I'm with Onrypt. 100%
The only thing i'd add is that it's said you shouldn't press the buttons whilst under. (except the frog) I do and haven't had a problem yet.
Like he says tho, a computer would be a better bet. Allot depends on the type of diving you're doing and how often you're gonna do it. Nice easy Caribbean rec diving like we do here barely warrants even a computer, unless you're accumulating for a few days in a row. Low viz colder water diving, then you want to take it very much more seriously.
No matter which way you go, nothing beats experience. In gaining that, it's very easy to over task load yourself. No offense, but if you're learning, the last thing you want to be doing is playing with your shiny new toy whilst you're in an unfamiliar environment that could kill you. In addition, you'll piss your instructor right off.
(I really dont mean to be condescending. I'm a DM, it really does piss instructors right off, trust me)
Good luck. It is a fantastic sport.
Dan.