The mail I mostly read from Plan 9 is hosted on Plan 9, but I've done IMAP with it as well.

-- Running imap with multiple mboxes (folders or whatever) did not work
for me (only one of them was updated).

This is almost certainly a configuration issue. It's not exactly clear what you mean by "wasn't updated", but I can't think of anything that matches my experience. Setup with plumber, faces, &c can take some thought up front, though.

-- Threading did not work properly.

Folks have put this into the readers, but I don't use it and haven't evaluated it.

-- When something went wrong during 'sending' from acme Mail, I did not
get any information that the mail had not been sent. So actually I always
had to control sending an email from, say, gmail's web interface.
(Or had to look manually into the logs.) That's a pretty bad behaviour.

That is bad behavior. I haven't observed (n)upas to be any worse in that regard than any other system I've used, though. Upas maybe provides one or two more places for the handoff to go wonky, but there's always a handoff that can go bad.

Regardless, if this is coming up with *any* regularity, I again suspect a configuration issue.

-- You can't easily search within all mail like you can using gmail
(for anything in the body, withing given dates, from somebody,
combinations, etc.

True. I wrote Mg (http://9fans.net/archive/2008/11/647) to offset some of these deficiencies, but "modern" interfaces are well ahead here.

-- I don't know how to correctly 'forward' an email from within acme Mail.

If you just care about sending the content on, open the message, edit the first line to who you want it to go to, hit Post. Fastest method, although you're tweaking the original. If you'd rather the original message be included unmolested, open the message, hit Reply, edit the address, hit Post.

-- the fact that gmail helps you to fill addresses when writing an email
is extremely handy and useful.

Agreed.

That's just a few things.

The main thing for me that prevents me from using it for more of my mail is the lack of a good HTML formatter. I occasionally get mail I actually care about (and often get mail that I don't) where the formatting matters. It's rare enough that I can punt that to other devices and have it be okay, but common enough that it's distracting.

Configuration, especially when all you're doing is the client (IMAP) side, is more of a pain that most other options.