Some differences between Roomba 880 and 620 that you may want to before buying the new 880.
Roomba 620 vs 880
images from manufacturer

Let’s look at some new things on the 880 compared to 620.

On-board Scheduling
The 880 has the on-board scheduling that the 620 does not feature. The scheduling allows you to preset the vacuuming time, then the Roomba will run at your desired time automatically.

Room to Room Navigation
The 880 supports the Lighthouse that helps navigate the Roomba room to room, this can make the Roomba vacuum multiple rooms more thoroughly and efficiently. While the 620 is not compatible with the Lighthouse technology.
If your house is small, then the Lighthouse function is not a big deal.

Bin and Filtration
The 880 is equipped with a larger dust bin to hold more dirt, and a HEPA filter to replace the standard filter on the 620 to trap more small particles and improve the air quality.

Dirt Detect Version
The 880 upgrades the Dirt Detect Series 1 on the 620 to Series 2.
The difference is that the Series 2 not only use acoustic sensor as the Series 1 has but also employs optical sensor to find dirt.
Furthermore, the 880 also features a Persistent Pass Cleaning Method that applies a back-and-forth cleaning pattern to clean where it finds excessive dirt.

Full Bin Indicator
The 880 also adds a full bin light to show you when it is full and needs emptying.

Home Base and Charging
The home base design is different now.
The one included within the 880 is an Integrated Home Base, it has no separate power supply, while the 620 comes with a Self-Charging Home Base with a Power Supply.

Battery
Both of them use the nickel–metal hydride battery.
The 880 however, upgrades the Advanced Power Battery on the 620 to a XLife Extended Life Battery that doubles the lifespan.

Included Accessories
The 620 only comes with a Flat Brush Cleaning Tool to remove hair and tangles from the brush while the 880 does not come with.
While the 880 comes with 2 Virtual Wall Lighthouses with C batteries included, 1 Extra HEPA Filter, and a Remote Control with batteries included.

Cleaning System
This is the key change between these 2 models.
If you own the 620, you can see that the 620 uses 1 Bristle Brush and 1 Rubber Brush in the bottom to pick up debris from the floor.
The 880 now does not use these brushes, instead it uses 2 Brushless Extractors.
And the 880 has the Airflow Accelerator design that creates a sealed channel that concentrates airflow to pick up more debris.
On top of that, 880 has an upgrade on the vacuum to generate more powerful suction.
iRobot claim that all the changes in the cleaning system will reduce maintenance and improve the cleaning results.

If you know more difference between Roomba 620 and 880, please share below.