I have a Treo 600 with Cingular service. I agree with the comments posted above.

The camera on the 600 is pretty crappy compared to the 650, and its missing some other features as well. On the other hand, I got the unlocked version really cheap, and I have a great digital camera, so it isn't as though I rely on the Treo to do anything other than be a combined Palm and mobile phone.

I recommend the Treo. I've heard good things about the Sony Ericsson P910.