I have the treo 600, infact i have had 4.
As a pda its good, because of the keypad, and the numerous palm software, and syncing with outlook , even software to connect your laptop to surf the web on the go, on the laptop with decent speed ...
As a phone it sucks....the speaker phone and sound is horrible. Its a lemon. The reason i had 4 so far, is because of a continuous problem with reception sound, they keep replacing it.
The 650 is better because it has better resolution, an improvement in camera quality over the 600, bluetooth......but nevertheless, i still read complains about the quality of the phone itself like the quality of the 600..
If you really have to carry a pda and a phone in one...then the treo is your best option for a combination of both....but if sound quality and size is a concern to you..then its not for you.....
Go here ...you will learn alot about it..and find all the answers you need.specially from consumer feedback..
http://reviews.cnet.com/Cell_phones/2001-3504_7-0.html?tag=glnav