I'm using old and battered GoPro on two of my FPV quads as the pilot cam and recording at the same time and also displaying OSD data, works fine. On the one that gets the most use I added a battery backpack, effectively doubling the battery life so concerns about running out of battery power are nonexistant. You can also leave the GoPro OSD turned on and you'll be able to see the battery status while you fly so either way it shouldn't ever be an issue as long as you remember to check the battery icon once and a while to see where its at they tend to fall off quickly once they get down to one bar.
One advantage is you get a better field of view from a GoPro than from a dedicated FPV camera, the big disadvantage is they suck in low light conditions and when you're flying towards a bright light source like the sun. I've gotten used to it so I just fly through it or turn a bit so the contrast returns. It is much easier to fly with a good CCD FPV cam as the view tends to be more consistant but a bit narrower, my other 3 FPV multis all have good quality FPV cams and those are the ones I rely on regardless of conditions.
This was the first FPV flight with my QAV500, the one I use the GoPro on exclusively and also the one with the battery backpack. Video here was a bit twitchy as I hadn't got the gains dialed in yet when this was recorded but other than the overlay of OSD data and a bit less resolution over the downlink the view in the goggles wasn't significantly differrent from what you see here...
I'm uploading one of the ground station recordings of the QAV in flight for a comparison, it will be up on YT shortly and I''l put the link here...
Here's what the GoPro looks like as an FPV cam through a set of Dominator goggles...
Ken