I've been a GTD user for about 3 years, and a Palm user for about 10 years. Between the ubiquity of my Treo and the efficiency of GTD, I feel like I'm pretty well set.
The only exception is that it would be nice to make use of the internet at any computer I'm at to quickly and easily make notes to myself. I understand Evernote does this. But I worry trying and using Evernote will just confuse and complicate the Palm / GTD system that's been humming along well for me over the last few years.