You will find that there are some distinct differences between Roomba 880 and 630 when comparing them.
Roomba 630 vs 880
images from manufacturer

Cleaning System
The cleaning system changes a lot.
The 880 now employs 2 Brushless Extractors versus 1 Bristle Brush + 1 Rubber Brush on the 630.
Brushless Design = less hair tangle, less maintenance
The 880 cleaning system also employs New Vacuum and Airflow Accelerator to delivers better suction to pick up more.

Filter
The 880 uses HEPA filter whereas the 630 uses standard filter.

Dirt Detect Technology
The 880 uses Dirt Detect Series 2 that employs both acoustic and optical sensors to find dirt whereas the 630 uses Dirt Detect Series 1 that employs only acoustic sensor to find dirt.

Room to Room Navigation
The 880 is compatible Lighthouse that helps room to room navigator whereas the 630 is not compatible with lighthouse.

Scheduling Feature
The 880 has on-board scheduling that you can use to schedule the unit to vacuum max 7 times a week whereas the 630 does NOT have the scheduling feature.

Full Bin Indicator
The 880 has a light to tell you when it is full whereas the 630 has no full bin indicator.

Persistent Pass Cleaning Method
The 880 employs a back-and-forth cleaning pattern to clean areas where it finds excessive dirt whereas the 630 does not have this Persistent Pass feature.

Battery
The 880 includes the Xlife Extended Life Battery to replace the APS battery on the 630, iRobot claim that the Xlife will doubles the time before you may need to replace the battery.

Home Base
The 880 comes with the new Integrated Home Base whereas the 630 comes with a Self-Charging Home Base and a Battery Charger.

Dimensions and Weight
The 630 measures 13.39 inches in diameter, 3.62 inches in height, weighs 7.9 pounds. The 880 measures 13.9 inches in diameter, 3.6 inches in height, weighs 8.4 pounds.
You can see that the 880 has a larger diameter and heavier weight.

Retail Price
The 880 retail price is much higher than the 630.
On irobot.com, the 880 is priced at $699.99 versus $349.99 on the 630.

Included Accessories
The 880 comes with 2 Virtual Wall Lighthouses with C batteries included whereas the 630 comes with 1 Auto Virtual Wall without C batteries.
The 880 comes with a Remote Control that lets you turn it on/off, steer it around, and remote access to Roomba commands, while the 630 does not come with this kit.
The 630 comes with 1 flat brush cleaning tool whereas the 880 does not include.
Both of them come with 1 Extra Filter.

Noise Level
The 880 noise level is slightly lower, but the difference is not significant.

If you know more difference between Roomba 630 and 880, please share your reviews below.

There are a few differences between Roomba 880 and 760, let’s compare these 2 Robotic vacuum cleaners below.
Roomba 760 vs 880
images from manufacturer

Cleaning System
The 880 changes a lot on the cleaning system, it uses 2 rubber Brushless Extractors to replace the Rubber Brush and Bristle Brush on the 760. With the extractors, there will be less hair tangle problems, so less maintenance.
There are also a new vacuum that generate more suction power and new Airflow Accelerator to help pick up more dirt.

Filter
Both of the 880 and 760 use HEPA filters, however, the 880 uses one large filter whereas the 760 uses dual small filters.

Lighthouse
The 880 is compatible with the Lighthouse which can navigate the Roomba to clean from room to room whereas the 760 does not support the Lighthouse.

Home Base
The 880 uses an Integrated Home Base for docking and recharging whereas the 760 comes with a Self-Charging Home Base and a Battery Charger.

Battery
The 880 uses the XLife Extended Life battery that has longer lifespan than the Advanced Power System battery that is used on the 760.
The Xlife battery costs $79.99 that is $10 more expensive than the APS battery, and the XLife battery can be used on the 760.

Full Bin Indicator
The 880 features a full bin indicator whereas the 760 has no light to indicate when it is full.

Dirt Detect
The 880 uses Dirt Detect Series 2 to find dirt whereas the 760 uses Dirt Detect Series 1.
The difference between series 2 and 1 is that the series 2 adds the optical sensors while the series 1 uses only acoustic sensors.

Noise Level
The 880 is quieter than the 760 according to user feedbacks.

Price
The 880 is more expensive than the 760.
The 880 is priced at $699.99 versus $449.99 for the 760 at irobot.com.

Included Accessories
The 880 includes 2 Virtual Wall Lighthouses (batteries included) while the 760 comes with 2 Auto Virtual Walls (C batteries not included).
The 880 comes with 1 Extra Filter while the 760 comes with 1 Extra Set of Filters (2 filters).
The 760 comes with 2 brush cleaning tools while the 880 has no brush cleaning tools, however, the 880 does not need these tools since it has no bristle brush.
Both of them come with a remote control.

Color
The 880 is in darker color.

Know more difference between the iRobot Roomba 760 and 880? If yes, please share below.

The major difference between Roomba 650 and 595 are the carrying handle and included accessories.
Roomba 595 vs 650
images from manufacturer

Carrying Handle
On the faceplate, you can find that the 650 adds a recessed carrying handle while the 595 does not have.

Accessories
The 595 comes with more accessories.
The 595 comes with 2 Virtual Walls with 4 Duracell C batteries while the 650 comes with 1 Virtual Wall without C batteries.
The 595 comes with 4 AeroVac filters whereas the 650 comes with 1 AeroVac filter.
The 595 comes with 3 Side Brushes, 3 Rubber Brushes, 3 Bristle Brushes while the 650 comes with 1 Side Brush, 1 Rubber Brush, 1 Bristle Brush.
The 595 comes with 1 Round and 1 Flat brush cleaning tools while the 650 comes with 1 Flat brush cleaning tool.

Price
The 595 has been discontinued, it was sold at $299.99 at Costco and the official site did not sell this model while the 650 costs $399.99 at irobot.com.

If you know more difference between the Roomba 595 and 650, please share below.

The difference between Roomba 760 and 650 are as follows.
Hightlights: Cleaning System, Filtration, Accessories
Roomba 760 vs 650
images from manufacturer

Cleaning System
They both use the iRobot patented 3-stage cleaning process to vacuum, however, there is still difference on the cleaning system, they 650 employs AeroVac version 1 while the 760 employs version 2.
As part of the version 2, the 760 uses a redesign cleaning head that is claimed to provide better performance for picking up pet hair, dirt, allergens, etc. They said the cleaner head is designed particularly for pets.

Filtration
There is big change on the filtration.
Unlike the 650 which uses a standard filter – AeroVac Filter, the 760 uses Dual HEPA Filters inside the bin. The HEPA filters do better at filtering fine particles as tiny as 0.3 microns, thus improve the indoor air quality.

Bin
The 650 uses AeroVac Series 1 Bin whereas the 760 uses AeroVac Series 2 Bin. Not much difference on the capacity.

Price
There’s a $50 price difference.
The Roomba 650 costs $399.99 whereas the 760 is priced at $449.99 at store.irobot.com.

Extended-Life Power Management
The 760 features the Extended-Life Power Management software which is claimed to deliver up to 50% longer battery life.

Remote Control
The 760 comes with a Remote Control to let you control the Roomba with no need to touch it while the 650 has no remote control.
However, this kit is not that practical, there are a lot of users do not use it even they have it.

Accessories
The 650 comes with 1 auto Virtual Wall, 1 flat brush cleaning tool, 1 extra AeroVac filter.
The 760 comes with 2 Auto Virtual Walls, 1 flat brush cleaning tool and 1 round brush cleaning tool, 1 Extra Set of HEPA Filters.

Dimensions and Weight
The 650 weighs 7.9 lbs versus 8.4 lbs on the 760.
The 650 measures 13.39 inches in diameter versus 13.9 inches on the 760.

Color
The 760 is in lighter color.

Here are some key difference between Roomba 770 and 650.
Roomba 770 vs 650
images from manufacturer

Cleaning System
The 650 employs the AeroVac Cleaning System whereas the 770 employs the AeroVac Series 2 Cleaning System.
According to iRobot, the 770 uses a new cleaning head that is designed with pets in mind, however, users say that there is no apparent difference on the performance.

Dirt Detect
The 650 features Dirt Detect Series 1 which use acoustic sensors to find dirt and debris whereas the 770 features Dirt Detect Series 2 With Persistent Pass Cleaning Method that uses acoustic and optical sensors to find dirt and debris, and spends more time cleaning spots where it find more dirt.
According to user reviews, the Series 2 works better than the Series 1.

Filtration
You will note the difference on the filter system when comparing them, the 650 uses 1 larger standard AeroVac Filter whereas the 770 employs 2 small HEPA Air Filters to replace the larger filter, HEPA filter can capture more fine particles and leave cleaner air behind.
The filter materials for the 770 feels better than that on the 650.

Full Bin Indicator
The 770 features a Full Bin Indicator that tells you when the bin is full and needs to be emptied whereas the 650 does not have this indicator.

Bin
The 650 uses AeroVac Series 1 Bin whereas the 770 uses AeroVac Series 2 Bin.

Extended-Life Power Management
The 770 has the Extended-Life Power Management software to deliver up to 50% longer battery life whereas the 650 does not feature.

Remote Control
The 770 comes with a Remote Control whereas the 650 does not include.
This kit is not that practical.

Accessories
The 650 comes with 1 auto virtual wall while the 770 comes with 2. The virtual wall is useful for keeping the Roomba out of off-limits areas.
The 650 comes with 1 flat brush cleaning tool while the 770 comes with 1 flat brush cleaning tool and 1 round brush cleaning tool.
The 650 comes with 1 Extra AeroVac Filter while the 770 comes with 1 Extra Set of HEPA Filters (2 filters).

Appearance
As you can see from the product pictures, they have different faceplate designs.

Price
The Roomba 650 is $100 cheaper than the 770 which is priced at $499.99 at store.irobot.com.

Dimensions and Weight
The 650 weighs 7.9 lbs, measures 13.39 inches in diameter, 3.62 inches in height.
The 770 weighs 8.4 lbs, measures 13.9 inches in diameter, 3.6 inches in height.

Both of them feature the on-board scheduling; but they do NOT support Lighthouse, so no room to room navigation.

Discuss below if you know more detail about them.