Press and hold the button for 10 seconds, and it'll come on. 5 second press reset your trip.

As for the computer itself... Lemme go find something I posted before.

I have one, and really, it's all looks. It's not particularly fancy, no fancy settings (I have the 12-mode one), after a while, it starts to look ugly instead of growing on you (for me anyhow). For the price, I wish I'd bought something else. But, it is easy to move around (which is convenient for me since I have too many bicycles) and it's waterproof with a light. It get's the job done, but maybe not for it's price.

I think the aesthetics of it are not kind to me because it's plain bulky, not sleek at all. But I have nothing else to compare to other than wired ones, so that's just me.