I'm on my second Treo. I had a Treo 600 and now a 700P (with the Palm OS). The 600 had one major flaw, which was that the battery was not replaceable, and all the memory was RAM. So, if your battery died, you lost everything you had in the phone. Can't tell you how many times that happened. The battery was lucky to last 24 hours, especially if I had a lot of calls I was making.
The 700P seems to have fixed all the problems that the 600 had. It's a bigger phone (which doesn't bother me). The battery lasts much longer and it's user replaceable. I love the keyboard, which I use for text messaging (I don't use the eMail feature). I also use the thing as an MP3 player, for which it works great. The Palm OS is my preferred over Windows Mobile.
I've played around with the BlackBerry phones and just haven't liked them as much. Then again, I had Palms in high school (the early ones), so I'm used to that interface. That is probably most of the reason. My friends who have BlackBerries love them, but the ones who used to have Treos miss the Treos.
The 600, for its faults, was still pretty durable. The 700P I've had for about 10 months now, and it hasn't given me a single problem. Cost is not great, though. I seem to recall mine was about $400 including case and a few accessories, with my work discount.