I use Thunderbird occasionally for newsgroup posting (news.povray.org, on occasion) and have used it a little bit for e-mail (only with my s.net account, really, because ISPs seem to love to provide their clients with absolutely terrible webmail interfaces). I mainly use Gmail for my stuffage. Google also provides a very nice cell phone client, so if you want to check your e-mail on the go (and have a data plan and J2ME capable phone at your disposal) that's the way to go. The mobile Gmail client supports the keyboard on my Blackberry 8300, so I'd say that if your phone has a keyboard it'll probably work with it.
I use Outlook for all of my work stuff. It's pretty nice for enterprise type stuff, but it's complete overkill if you just want to use it as an e-mail client.