Sometime back (I don't know when, it just popped in front of me today), Lifehacker conducted a poll with it's readers, asking them whether they use a desktop-based email client or web-based.
Result? 54% like web-based.
Surprising? I don't think so. I know if I use Outlook express, I won't get the conversation view in Gmail, I'll have to reconfigure it everytime I reinstall Windows (which is - well - whenever it's half-life has reached)
See this Lifehacker post for more.