The Treo 650 arrived yesterday and I spent about 3 hours playing with it last night. I have read a lot about resets and other issues, but so far it has been performing flawlessly. Here are my initial impressions on the improvements over the 600; better display obviously, larger keys arranged in a more comfortable arcing pattern, much brighter keyboard backlight, seems a bit snappier, phone calls sound a bit clearer, Bluetooth rocks with the 3 headsets I tried so far, reception seems better (I actually received a good signal in my house to surf and I never usually do), the camera is a thousand times better (check out my SplashBlog photo blog to see some differences) and shots appear much clearer on the device, and I like the new hardware buttons and placement. The 650 is also slightly less wide than the 600 which just improves the feel in your hand.
I am placing my order for a Treo 650 today on the palmOne site and my new GSM unlocked Treo 600 may now be going up for sale. However, my wife said she really liked the Treo 600 and it would be a very nice phone to have as a backup since I did get it for $300 and it is new. If anyone is interested in buying it for around $275, just email me at palmsoloATgeekDOTcom to discuss. It is a new replacement from palmOne and is still in perfect condition since I have only had it a week.