Yes. That is the point. It is possible that I won't spend time implementing minor things that none of the popular mail clients use (I'm not after the spec-junky-of-the-year-award). I'm about to begin refactoring the mess that I created into the real packages. From then on you'll be able to watch the progress. Before I left last week I'd successfully gotten Thunderbird to download mails. My plan for next week is to get them flagged, deleted, and whatever else I can do in Thunderbird. I've also upgraded to T-Bird 1.5 and I'm hoping that its IMAP sucks less (it misses the whole point of IMAP).
Anyhow, you should be able to use Thunderbird with M5 when it is released. Other clients might work but I'm only promising Thunderbird for M5. It will also be a bit hacky.
Following M5, I'll round of the edges.
My hope is to have 1.0 working with SpecIMAP, though it is possible that I won't bother with bizzare permutations of commands that clients don't use if it seems not worth the time.