Not a fan. First off, I can't see through my thumb... or finger. Second, I'm sure once you memorize the locations of the characters it would be easier, but that would include letters, numbers and symbols. I'm not opposed to the idea of using a non-qwerty input, but I find that the pullout keyboard on my D1 makes this more hassle than it's worth, SlideIT works great for my touchscreen keyboard when it's necessary, and Vlingo is rapidly becoming my favorite way to communicate when I'm driving.
Not to mention, SlideIT has a free version and Vlingo is also free. Maybe if they had a beta or trial for you to get used to, people would get the hang of it and be willing to pay for the full version. However, since I've only got 24hrs to evaluate it, I think it'll be forgoing any future 8pen experiences and getting my money back.
I'm not saying this is a bad app, or a bad idea... just not right for me.