I use Thunderbird day-to-day. It has a lot of faults, but it's straightforward and simple.

The thing that pisses me off most about Mail.app is that it decides that it needs to combine all of my Inboxes into one view. I really don't want my personal mail mixed up with my work mail. Yes, I know that it doesn't "save" it that way, but I still want to keep my lives separate.

You're right that Outlook is a piece of garbage, though. I'm forced to use it for corporate calendaring. My main complaint with it is it defaults to horrible-netiquette mode, encouraging everyone to quote and forward wildly improperly. (Someone should write a virus that fixes people's Outlook settings.) My favorite new bug is that if you send to an Exchange mailing list, it goes ahead and expands it into all the individual users. So if you're trying to search for that email you sent to a group; too bad.

Surprisingly, I end up using MS Entourage as my MacOS mail client. It has a few things that bother me, but it mostly does things the way I want or lets me easily change its behavior with easy-to-find options. (Someone on MS's Mac UI team really needs to have a talk with folks over at the Windows UI team.)