If you want a really detailed and very accurate review, see Ifti Bashir's Youtube review. https://www.youtube.com/watch?v=Lt8ce5FVi5QAnother great review from Ifti! I just tested mine today on my kayak. Everything Ifti says is absolutely on target, the kudos and the criticisms.
It works as advertised and it works well, except that the membrane on the back isn't quite as sensitive all over as the LCD bacpac itself, but that's not unworkable. Easy enough to use, and in some ways easier than the phone app.
The cost, however, is really $180 USD plus tax to get both the P1 and the LCD, which I didn't already have. So the real question is whether this is money well spent.
For my use around salt water, the phone app always made me nervous and wasn't as convenient as having a unit right on my wrist. Also, given the constant contact with water, their R1 model didn't fit my needs, as it was likely to get wet and corrode. (Otherwise I could have saved the $80 for the LCD.) The P1 is well sealed, as long as you're not trying to use it for diving or below 1 meter.
Given how I mount the camera on my kayak, there is no way I could have done the shot framing with the LCD on the Gopro itself. This is the only viable option I've found so far.
YMMV, of course!