X-Gadget has a Treo 600 vs 650 comparison review up. Summary: * The sound quality is slightly worse on the 650, but still better then most phones. * The screen is vastly better. * The camera is much better. * The CPU is a lot faster. * The software is slightly improved.
The reviewer didn’t mention low memory issues or spend more then a few minutes playing with Bluetooth. All in all, his take seems to be that the hardware is good, but PalmOS is getting long in the tooth and needs updated.