My main gripe had to do with too many open connections.. I have a lot of folders and sooner or later, if you access too many folders, Mozilla and CourierImap loose track of who's on first. Basically, you have to go offline and then back online to see what's in your folders.
Mozilla also seems to have problems with Courier, INBOX.folder notation for folders. Courier wants to put trash, sent, etc within the INBOX namespace, while the display likes to have those folders allongside the indbox namespace, so the defualt IMAP install aloways has to be adjusted to work with Courier-- not too big of a deal but confusing when trying to sell to non-tech people. These problems are worsened when you have more than one IMAP account running. For instance, I have a Exchange IMap account and a Courier Imap account. It doesn't take long for Mozilla to cache too many connections and freak Courier out then ... and it seems to ignore the cached connections cariable in prefs.js then.
I thought these bugs were still active in 1.2 -- I haven't got a notice and I was subscribed to them. Now it looks like they may be fixed !! That would be good news indeed. I guess I'll have to grab the nightly and see what happens.