I've had the Treo 650 for months, and agree with most other raters that it had problems.  It reset spontaneously, had 50% less usable memory than the 600 and the phone quality was not as good.  Palmone has updated the ROM now and fixed all these problems.  My usable memory went from 5 mb to almost 11mb, and the phone is now completely stable.  No need for reservations anymore.  The ROM update is not yet available for Cingular, but will be, I suppose, eventually.