Update: I’d like to address that it was recently discovered that the 4000/400 series not having the sensors that tell when the wheels are down is actually not true, as it seems all others have these sensors. My theory is they left them out on mine since it’s remanufactured.
This video is an in depth video explaining the similarities and differences in each roomba series that has been released. From the very start, roombas have been extremely reliable in many people’s homes, so take a look and see what changes this remarkable vacuum have been through as the years have gone by. Feel free to point out anything I missed in the description, as I bet I’ll get a lot of comments about that, haha. This video contains info about:
-Original Series
-400 & 4000 Series
-500 Series
-600 Series (Including Professional Series)
-700 Series
-800 Series
-900 Series
-E Series
-I Series
-S Series

Vacuum Wars did a great comparison on specifically the original and S9+, check it out here!    • The Original Roomba (2002) vs Roomba ...  
For anyone interested in refurbished 900 series robots, check out my eBay store! https://ebay.com/usr/misbrisco