I've now had my Cingular Treo 650 for two weeks, it is without a doubt worth every penny.
The 650 is a HUGE improvement over the 600. Among other upgrades it offers a much better screen (compared with the 600 it is the equivalent of switching from a black and white television to a 52" plasma HD-TV), bluetooth, faster speeds using EDGE, a much nicer backlit keyboard and a much improved camera.
On the downside, it is incredibly unreliable. It seems that it either locks up or reboots itself several times per day. The folks at TreoCentral.com claim that a firmware update is on the way, I'll keep my fingers crossed. In addition, bluetooth dial-up networking is missing from the bluetooth control panel (though www.shadowmite.com has an unsupported version of the bluetooth application that does enable DUN, use it at your own risk).
Overall I'm happy with the 650. As a previous 600 user I already had the "how did I ever live without this?" feeling. Being able to access mail, calendar, and contacts from anywhere is an amazingly useful capability. The 650 just adds to the enjoyment. With the improved screen as well as the bundled "Documents-To-Go" application, I can now easily create, edit, and read Word documents as well. If you are a road-warrior or you just want mobile access from anywhere at anytime, the Treo just might be the solution.