I have an iPhone and it's REALLY fast for me, but that could be because I use it 24/7 :P It did take a bit of getting used to though, the first couple times I tried to text, I kept missing the space bar so it would turn out like doyou wantto goto themalltomorrow? And everyone was like ..what? Lol X3 but otherwise I think it's pretty awesome.
Have you guys heard of swipe phones? These phones have super-fast input method for texting. I heard its now holding the Guinness World Record for fastest way to text. Here's a link for preview: http://funadvice.com/r/3k5244tc8j
To me any phone with a full pullout keyboard seems to be the fastest. I have a samsung impression and i can text as fast as i type.