I've got a Treo 600 and absolutely love it. I use T-Mobile, so though the plan is pricey, it includes unlimited internet. I can even trade stocks while driving to Omaha.
Dave's got a Treo 650, which is supposed to have some advantages over the 600. Of course, it came out shortly after I received my 600 after a wait of about 3 months. Dave was able to get his 650 pretty quickly.
One thing I haven't tried with it yet (but will) is the adaptor that'll let my laptop get on the internet through it. That could be really handy, although its internet speed feels far slower than even dialup.
Fortunately, places like yahoo.com and scottrade.com have a "mobile." server, which works wonderfully for the low bandwidth. I'd like to do something similar with our message boards.