I hate defend Microsoft, but personally I found that Outlook Express to be one of the (overall) best IMAP clients out there... (on windows at least). Mulberry never quite seemed to work the way I expected and is now discontinued.
I personally used OE for many years and it took quite a long time for Thunderbird to work well enough with offline/disconnected operation to be useful *for me*. Outlook also seemed to work ok in most guises from 2002 ish onwards (although 2003 seems a great improvement in most ways).
All clients seem to have bugs and problems, but personally I rate OE as one of the best clients out there for people on dialup and lower bandwidth lines. I particularly like the popup box which tells you *exactly* what it's doing, how long is left, and also makes it very simple to hangup the dialup connection when finished (or not). It has very precise syncing of folders, so you can avoid downloading all messages over your expensive GPRS link, and you can very precisely choose even which indiivdual messages to sync. If it's possible to make Thunderbird auto dial and hangup, and show the same progress dialogs then I'm all ears... In the meantime we put all our customers on OE (or Windows Mail as it's called under Vista) - works very well in practice
Good luck
Ed W