Graffiti is pretty easy once you get the hang of it. Great for short notes in the field but I wouldn't want to write a novel using it. I also bought a keyboard to use at home/office and it speeds thing up quite a lot.
As to my handwriting starting to look like Grafiti, well it always did!
Hehe... if your handwriting is very bad, learning accurate grafitti may help improve it! ;) But really, I don't think there's much variation in sensitivity/accuracy of grafitti from one model to another.
As far as on-screen writing/doodling programs I've tried, the variations in accuracy/smoothness are in the software itself.