I can't speak for Mutt, but I've been using UW IMAPD and Netscape 4.7x (and its predecessors) as my mail client for years. The way to think of IMAP is as a remote filesystem. It has directories and files, only those are now called folders and messages, for some reason. IMAP daemons (file servers) seem to have two different ways of storing messages. Some, like UW IMAPD, allow you to store your folders as traditional Berkeley mbox-style flat files. These are quite portable, but they're excruciatingly slow to work with when they get large. UW IMAPD and Cyrus support nifty database-like storage that speeds things up immensely, but nixes any backward compatibility that you might otherwise want.
On the client side, based on my research, the clients worth talking about are Microsoft Outlook, Qualcomm Eudora, Netscape/Mozilla Communicator/Messenger, Ximian Evolution, and Emacs Gnus. I'm leaving Mutt off the list because I'm not seeing much in the way of multimedia message support beyond basic MIME attachment handling. Gnus can, at least in theory, integrate with Emacs W3 mode (although I haven't figured out yet how to make it happen, or how to view previously-read messages, either -- the Gnus manuals suck).
Outlook, Eudora, Netscape, and Evolution are all spiritually related. You get a "three-pane" GUI, with your mailboxes down the side, the messages in the current mailbox on top, and the current message everywhere else. HTML content is handled in the obvious way, and MIME attachments can be viewed inline. All of them have filtering and auto-refiling support which isn't wonderful but doesn't suck. Gnus is better in some respects, but worse in others. (Emacs Mew totally rocked for refiling rules, but it doesn't have IMAP support yet.) I've been running Evolution for a few days now, and I'm quite impressed. In many respects, it's much faster than Netscape or Eudora, although it's not as stable yet. Evolution also has some fairly nifty search-engine-like features built into it.
I'm still not totally happy with my setup. Maybe I need to try the latest Outlook from Office XP. Hopefully it's got better IMAP support than Outlook 2000, which consistently crashed before it could finish scanning all my folders.