The major decision regarding the Treo is whether to use Palm or Windows software. There seem to be pros and cons of both systems. A lot has been written on the subject, especially with regard to the Treo 700.
One pretty good review appears here: http://www.pocketnow.com/index.php?a=portal_detail&t=reviews&id=853
The review breaks down the different features of each and offers comparison and commentary.
Here is another article: http://www.technologyevangelist.com/2006/09/the_treo_700p_vs_tre.html
It appears that Palm is the simpler, more stable, more user-friendly of the two systems. While Windows adds some bells and whistles and gives you better access to documents and programs, there has been a complaint of it crashing often or even regularly.