I've got a Handspring Visor. It took me a while to justify buying it in the first place but it's been money well spent.
The "HotSync" function is very good. I commute into London by train so if there are any E-mails to write then it's always handy to do them on the journey and then send them off as soon as I get in to the office.
The date book and the "Avant-Go" browser are pretty good as well.