I just saw this post by Tim Bray on email clients on OS X. I thought the timing interesting because I've just switched back to Mail.app from Thunderbird (on OS X). I switched to IMAP a while ago (and have never looked back), and for some reason thought that TBird was a better IMAP client. But a few days ago, I needed to import an mbox, and I have to admit I just couldn't figure it out in TBird. I started Mail.app, sat back while it synched to the server and did it's digestion (redline that core duo!), imported the mbox. I also read some mail.
Problem was, when I went back to TBird an hour later, I thought the same thing that Tim did - it does feel slower, and wasn't as visually appealing, etc. So I'm back on Mail.app for a while. It doesn't seem to have identities (which I think is a big flaw, but I can work around it), but otherwise I'm happy.
This is a validation of at least one reason why I chose IMAP - freedom from client. When I tried (and failed) to switch from Mac to Windows last year (because I was tired of carrying around multiple laptops...), I realized that Mail.app had it's own mbox format, and I vowed never to be beholden to one mail client again. IMAP lets me do this - I can use the best mail client for the platform, or even switch now and then to keep things fresh. Change is good.