I upgraded to a Treo 650 recently with the Palm OS. Overall very happy with functionality and speed and the battery is quite good. The one issue was that it would randomly crash about once a day and you couldn't tell when it did until you tried to use it. (Phone calls would go to voicemail) To compound the issue, you have take off the battery cover physically to do a soft reset!
After a bit of trial and error looks like the issue can be largely avoided by taking the Keyguard feature off. Treo responds from standby mode much faster (1 sec vs. 2-3) and it hasn't crashed yet. The risk of an accidental call is quite low since only two buttons can activate the Treo from standby mode.