I'm looking at the iBike and giving my Garmin 305 to my wife. I don't have the iPhone, but do have the iTouch.

As for the battery drain, the thing has it's own on-board 7 hour battery. So that shouldn't be an issue.
The Power meter version is expensive, like $749... so that's out.
They are running a special for $199 for the regular computer (HR and Cadence are extra... trying to find the price of these)
I eventually will be getting an iPhone, but it doesn't look like it's iPhone OR iTouch compatible... it's one or the other. That's not good, I'd hate to buy one and have to buy it again for the iPhone.

Still thinking.