I have the Treo 650 as well and I love it. Cingular is my provider and I'm sure they are a little more expensive then some of the other providers but it works great all over the world. (Fewest dropped calls ,

) I also use Log Book Pro and I haven't had any problems with it.
I use it for my Schedule, Email, Airline time table, MP3 Player, Logbook, Camera, World Clock, Games, Internet, Alarm Clock and obviously, Phone.
There is a great program called Express that keeps track of Stocks, Sports, Movies, News, Directions and 411 information all in one program that updates everyday.
I don't think you will go wrong if you get the Treo 650.
Good Luck!!