I have tried it all... folding keyboards, graffitti 1, graffitti 2, Fitaly, and the thumboard on my Treo 650 smartphone.
I rate them in the following order for speed:
1. folding keyboard
However, I rate them this way for ease of use (on the go):
4. folding keyboard
They are almost opposites. In other words, in my experience speed is inversely proportional to ease of use. In my job, I have to bring out my Treo, type a couple words or a note or Action item, then put it away. The thumboard is indispensible because I can enter items without pulling out the stylus. However, writing long emails or notes is difficult. That's why I carry a folding keyboard for those times I have the time to write something longer.