Dovecot performances are very good in comparison with courier. With that switch, we dropped the CPU usage of the server from 25 % to 10 %, and it's damn faster now. I have no idea why, but I think that it's better written looking at the code, and also that its usage of index files helps a lot.
We got no problem getting things work with public folders either, so the switch was almost painless.
The only problem we had is that Dovecot is too smart for some MUA. Consequently, we hit an 8 years old Mutt bug #969, which I also reported to the Debian BTS as #549204 with a not-well-tested-but-seems-to-work patch.