ill say this.. i use one alot nowadays.. and i think it takes alot of time to train yourself how to use it. Ive worked in offices where people did mechanicals.. in quark strictly with tablets.. i have to make one major point which has not been made.. IT IS A SHITLOAD EASIER FOR PEOPLE WHO ARE RIGHT HANDED...
if youre left handed...all the key commands become complicated..
as for drawing in illustrator..
ive read quite a few tutorials on inking.. (for comics) in illustrator.. its ok.. but its still a pain in the ass.
I think its very useful depending on what kind of person you are and what kind of graphic/visual applications youre doing.
i did this the other night..in a few hours.. just so people can get an idea of what someone whose spent a year trying to use it can kinda do.