I'm not too bothered by touchscreen keyboards because I tend to find it quicker to stop for a moment and use the keyboard. But anyway, to see how difficult it was to write I opened up the sketch app on my N900, and proceeded to write one handed with my thumb, and it wasn't difficult at all, so something like Graffiti* which uses slightly modified handwriting should work quite well one handed with a thumb.

The reason I wouldn't want to learn something like 8pen is that it'd take far too long to learn for the little use it'd get whereas something like Graffiti would be very quick to learn since it is based on regular handwriting. When learning something new that will take up a substantial amount of time to learn you have to consider what benefits there are to you and if it is actually worth the time investment. For people who write lots on a touchscreen only device 8pen may well be worth learning, for most people it won't. For me, I'd be happy with a decent T9 implementation on the N900 for short messages, and the keyboard for longer ones.

*Graffiti is now available for free on Android for those are interested and didn't already know.