1. It occasionally doesn’t properly return to the home base, because it’s hard to find a good place for the home base where it can’t be moved/knocked over accidentally. This happens maybe once every 2 or 3 weeks, and is just a mild annoyance: you just need to carry the Roomba to its home base yourself.
2. After 3 years of usage, the battery died in such a way that it emptied rapidly enough for the Roomba to not be able to return to the home base. This was easily fixed by installing a new battery.
I recently upgraded to the Roomba 980, and it seems to work better than the 780 — it cleans more quickly and returns to its home base more reliably. Also, less cleaning required due to a different brush design.
Overall, I feel that I’m definitely getting my money’s worth out of the product, and it’s doing a great job at vacuuming. YMMV, but for me, it’s great.