The 700p improves on the 650 by offering support for EV-DO (which means it will only work on Sprint and Verizon's networks for the time being), a slightly better form factor, an improved camera, more RAM and a few other tweaks. Perhaps the biggest benefit for business users is that the 700p now supports Microsoft Active Sync, meaning that the Treo's mail/calendar apps can be kept synchronized with an Exchange server, a feature that was previously only available on the 700w. Tom Keating over at TMCnet has a very detailed review that tells you everything you wanted to know about the 700 and then some.
I've been a Treo user since the 600 came out. My 650 is a little over a year old and still meets my needs though the reliability is still an issue (it resets itself about a third of the time i make a call using my bluetooth-enabled car kit, but I think that's more a function of the e-mail app I use [an older version of Chatter]. Still, the 700p is tempting, but I'll continue to wait until a GSM model is available.