The difference between iRobot Roomba 645 and 650 are as follows.

Roomba 650 vs 645
images from manufacturer

On-Board Scheduling
The iRobot Roomba 645 which is a Sam’s Club exclusive is a similar model to the 630. It has NO on-board scheduling whereas the 650 features the On-Board Scheduling that allows you to preset the cleaning time for Roomba to clean whenever you want.
You can see this difference from the faceplate of the two units.

Battery
The iRobot 645 uses the Xlife Extended Life Battery whereas the 650 comes with the Advanced Power System battery.
The running time of these batteries per charge are the same, the Xlife battery however, has a longer overall battery life.

Side Brush
The 645 uses a new easy to remove side brush that has a flat head screw while the Roomba 650 uses the brush with a crosshead screw.

Price
The 650 is more expensive.
The 645 is priced at $329.98 at samsclub.com whereas the 650 is priced at $399.99 at irobot.com

Brush Cleaning Tools
The 645 comes with 2 brush cleaning tools ( 1 round and 1 flat), while the 650 comes with only the flat brush cleaning tool.

Color
The 645 faceplate is in gray color whereas the 650 faceplate is in black color.

If you know other difference between the Roomba 650 and 645, please share below, thanks.

As far as I know, the key difference between iRobot Roomba 655 and 645 is that the 655 features the on-board scheduling whereas the 645 does NOT have the scheduling.

Extra Accessories
The iRobot 655 comes with 1 extra Side Brush, 1 extra Bristle Brush, and 1 extra Rubber Brush whereas the 645 does not include extra brushes.

Price
The 655 is $20 more expensive than the 645; the 645 is a Sam’s Club exclusive unit, while the 655 is sold at Costco.com.

If you know other difference between iRobot 645 and 655, please share below.

Here are the difference between iRobot 650 and 655 :
iRobot 655 vs 650
images from manufacturer

Battery
They come with different battery.
The iRobot Roomba 655 uses the purple XLife Extended Life Battery while the iRobot 650 uses the Yellow Advanced Power System Ni-MH Battery.
According to iRobot, the Xlife battery doubles the time before you may need to replace your Roomba battery.

Side Brush
The Side Brush on the 655 has a new design that make it easy to remove whereas the 650 comes with the original Side Brush attached by a Phillip’s Head Screwdriver.

Charging Kits
The charging base is different.
The iRobot 655 comes with the Dual Voltage Integrated Home Base while the Roomba 650 comes with a Battery Charger and Home Base (110V).

Cleaning Tools
The 655 comes with 1 flat brush cleaning tool and 1 round brush cleaning tool whereas the 650 comes with only the flat brush cleaning tool.

Extra Accessories
The 655 comes with 1 extra rubber brush, 1 extra bristle brush, and 1 extra side brush. While the 650 does not include these extra brushes.

Price
The retail price of 655 is $349.99 at Costco while the 650 is priced at $399.99 at irobot.com

Color
The 650 is in darker color.

Both of them feature the On-Board Scheduling and include 1 virtual wall.

If you know other difference between the iRobot Roomba 655 and 650, please share below to help others buy the right one.

We will list some similarities and difference between iRobot Roomba 790 and 880 within the below table.

iRobot Roomba 790 vs 880

	iRobot Roomba 880	iRobot Roomba 790
Color	Grey / Black	Silver / Blue
AeroForce™ Performance Cleaning System	Yes	No
Tangle-Free AeroForce™ Extractors	Yes, 2 counter-rotating rubbery bristle-free extractors grab and breaks down dirt and debris from different types of floors. The new design of the extractors eliminate the tangles and jams of pet hair, human hair and other debris.	No, 790 use 1 bristle brush and 1 flexible beater brush to grab hair, dirt and debris.
Control Panel	Buttons	Touchpad
Similar Features	HEPA Filter, Dirt Detect™ Series 2, support Room-to-Room Navigation, iAdapt® Responsive Navigation Technology, Self Docks And Recharges, On-Board Scheduling, Self-Adjusting Cleaning Head, Full Bin Indicator, Edge-Cleaning, Persistent Pass Cleaning, Spot Clean Mode, Carrying Handle, Power Management
Dimensions	13.9 inches in diameter, 3.6 inches in height	13.9 inches in diameter, 3.6 inches in height
Weight	8.4 lbs	8.4 lbs
Warranty	1-Year Manufacturer's Limited Warranty On Robot, 6-Month Manufacturer's Limited Warranty On Battery	1-Year Manufacturer's Limited Warranty On Robot, 6-Month Manufacturer's Limited Warranty On Battery
Battery	Roomba 880 features XLife battery that claims to have longer life than the battery on 790.

If you own these 2 robotic vacuums and know more details about them, please share with others below, thank you.

The iRobot Roomba 880 is latest Roomba robotic vacuum model.
It features the new AeroForce Performance Cleaning System which use 3 technologies to improve the cleaning:
Tangle-Free AeroForce Extractors: these 2 rubbery Extractors that are brushless replace the bristle brush and flexible beater brush on 770. iRobot claim that these Extractors pick up more from floors and require almost no maintenance because they eliminate tangled hair.
Airflow Accelerator: it helps concentrate airflow between the Extractors to draw in more debris.
High-Efficiency Vacuum: iRobot claim that this new vacuum on Roomba 880 provides a 5X improvement in air power than the older Roomba models including the 770.
Roomba 880 vs 770

Price:
iRobot Roomba 880 list price on irobot.com is $699.99 while the price of Roomba 770 is $499.99. (prices on 02-14-2014)

Battery:
iRobot Roomba 880 use the XLife Battery that claim to optimize the cleaning efficiency, result in delivering twice as many cleaning cycles as Roomba 770 over the life of the robot.

Bin:
iRobot Roomba 880 features a larger bin which can hold more dirt than the Roomba 770.

Lighthouses:
iRobot Roomba 880 support the Lighthouse technology that allows Roomba to vacuum from room to room while the 770 does not have this ability.

What’s in the box (when you purchase the robots from irobot.com):
iRobot Roomba 880:
1 iRobot Roomba 880
1 iRobot XLife Battery
2 Virtual Wall Lighthouses (batteries included)
1 Integrated Home Base
1 Remote Control (batteries included)
1 Extra HEPA Filter

iRobot Roomba 770:
1 iRobot Roomba 770
1 Rechargeable Battery
1 Battery Charger (3-hour charge time)
1 Compact Self-Charging Home Base
2 Auto Virtual Walls (each requires 2 C batteries, not included)
1 Extra Filter
2 Brush Cleaning Tools
1 Remote Control
1 Instructional DVD

Similar features:

• Fiter: HEPA filter;
• Dimensions: 13.9 inches in diameter, 3.6 inches in height;
• Weight: 8.4 pounds;
• Warranty: 1-Year Manufacturer’s Limited Warranty On Robot, 6-Month Manufacturer’s Limited Warranty On Battery;
• Dirt Detect Series 2 with Persistent Pass cleaning method;
• Control Panel: button control;
• iAdapt Responsive Navigation Technology;
• Self-Charging;
• Cleans On Schedule;
• Edge-Cleaning;
• Full Bin Indicator to tell you when to empty the dust bin;
• Spot Clean Mode to clean a small area quickly;
• Power Management to deliver longer battery life

If you know other difference between iRobot Roomba 770 and 880, please share with others below.