Glad I found this website--it's very helpful, especially since, although I've owned Roombas for several years, I know very little about robotics! I just took apart a couple of Roombas for the first time, using a link to disassembly instructions I found here, and found that it wasn't as bad as I'd expected, at least for a blue Scheduler and a white Discovery--four screws to remove the bumper plus a cable to disconnect, then 8 screws to remove the case plus another cable to disconnect. On both I also had to remove the control button panel, just 3 more screws.
Up to now I've restricted myself to thoroughly cleaning Roombas by taking out the brushes and checking carefully for hair and thread wound around the axles, and that usually fixes the circle dance. However, that technique was inadequate for these two, both of which had been given to me by people who'd given up on fixing them after probably several years of heavy use--they're both pretty beat-up looking. The Scheduler had the "dust bin not filling problem" due to the brushes turning only sporadically and the Discovery, which had been working okay for me for a couple of months, started doing the stop-start tango--about 4 stop-start cycles with brief movements, then stopping entirely. To make life more difficult, neither had working control buttons, so I couldn't try running the diagnostics. (I was still able to run the Roombas for vacuuming using the remote, a workaround I haven't seen mentioned here.)
I took the Scheduler apart first and when I removed the bumper I found that it was full of fine hair--no big surprise because there'd also been a lot of hair on the brushes and roller and in the wheel wells when I first got it. When I took the case off I found the innards completely full of hair and a heavy layer of hair and thread wound around the brush motor drive shaft--definitely a likely cause of the dust bin not filling.
The Discovery, on the other hand, looked quite clean in the wheel wells and I had long ago cleaned the brushes and roller. With it I suspected the bumper sensors, so I took off the bumper and everything I could see at that point looked a little dusty but with no sign of hair. Now comes the surprise--when I took off the case, this Roomba had just as much hair as the Scheduler, if not more, lodged throughout the interior! In fact it had twice as much hair wound around the brush motor drive shaft! I would never have guessed that even after taking off the bumper.
If I'd had an air compressor handy, I might have been tempted to save time and effort by just blasting compressed air around the bumper sensors instead of taking the Discovery apart, a shortcut which would have turned out to be a very poor choice in this case!
I haven't finished working on either Roomba yet, so I can't say how successful I'll be in repairing their problems, but I'm much more confident of success now that I've removed all that hair from each of them! To be honest, I'm surprised that either of them ran at all after finding what I did when I took them apart.
Moral of the story: If a Roomba has unusually severe problems, it will probably pay to go the trouble of disassembling it in case, like my Discovery, outward appearances are misleading.