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.