I think this question has probably been answered a million times on this board and I think the crowd here is obviously a little biased! ;)
But, being a Treo beta tester and finally leaving after the 650, I would never go back...there is no substitute for the instant email and the bulletproofness (nice word huh???) of the phone.
Of course it depends on what your goals are in a device though!
"I would like to meet the guy that invented beer and see what he's working on now!"