I just tried this and got it working well right away. At first I was making loopy arcs, but I quickly realized that you need to go directly (in a straight line) to the next letter you want. As long as I do that, it works perfectly. Even when Im sloppy, it guesses well.
I used it to send texts and emails, no problems with the interface and I like that its built in so (unlike some of the other apps) you dont have to copy and paste.
Swyssa about ThumbSlide For iPhone, v1.0.4