I used Thunderbird, and Eudora before that, before switching to Gmail. I figured it would be shortlived because I'd miss having a client. Instead, I've never looked back. Gmail does more than a client could and is so bloody intuitive. For the search function alone, it's worth it, but then the way it organizes messages...oh, fantastic!

Outlook just sucks so much, but my parents refuse to give it up. They email me and say, "We need new email addresses because we're ditching Comcast and we couldn't get Outlook to work with Gmail. What should we try?" and I want to write back and say, "You know, if you'd give up that damn Outlook BS, Gmail would be perfect, but you never listen to me so why do you keep asking??" So of course, they set up Hotmail. Whateeeeevvvveerrrrrr...