I would perfer a form that lasted a bit more then 20 or 30 seconds.... I certainly don't mind a time limit.... but why so short? (assuming it is something like 20 or 30 seconds).... Why not let last a minute or two? Or untill combat is over with? and then increase the timer a bit to make people use the demon form more tactically (rather then RELYING on it).... that way it's a good "oh crap those mobs are gonna be tough" or "oh crap those mobs are TOUGHER then I thought" button..... or if your in a raid and things start going badly, push the button, transform into a kick butt demon to even the odds for a bit.
Or perhaps they can make it sorta like a rage thing, but instead of just getting mad it has to do with enemies you've slain? (as a tribute to the demon you've maken a pact with?) so that a "meter" fills up and when it's full you can unleash the demon by letting him "possess" you.

Dunno, just throwing stuff out there... just saying I think a very short timer will be more annoying then helpful/cool. I mean it takes a bit of time to transform I would assume.... if it took 5-10 seconds just to transform, and it only lasts for 20 seconds, is it really worth it? Could you have done the same amount of damage by that time without transforming? (after you both transform, fight in demon, then transform back)