I'm using the mozilla client in mozilla 1.2 with yahoo mail - only prob is its a bit slow when i get lots of messages - am experimenting wqith hotopop at the moment too.
I use Outlook Express but only because it's all I know. The way it is so vulnerable really puts me off though. Tried Mozilla but didn't like the way it handles newsgroups and Incredimail doesn't do them at all so I'm stuck with OE