Pros: Palm OS, can do just about anything
Cons: Palm OS, unstable, bulky
I've used the Treo 700p for about two years now, and while it is a good, solid phone, but today's standards (2008), it is pretty outdated and slow.
It uses the Palm OS, which is known for its simplicity and ease of use, and it lives up to its name on that front. There is a huge base of software out there that you can install, which means you can make this phone do just about anything you want.
However, Palm hasn't updated their OS in nearly two years, and so it doesn't support many modern functions, including the ability to multi-task well. The Treo 700p is also prone to crashing; Palm has (reportedly) fixed some of these issues in later versions of the Treo line (such as the Treo Centro).
All told, I wouldn't recommend the Treo 700p (or even any Treo) to someone looking for a phone today. There are better, more modern options.