While racing I had loads of problems with every one I've had, but I've also spoken to a few people who at one point were sponsored by the company, using around 40 cameras at a time. They're known to corrupt memory cards, which is one of the main issues, but also to switch off for what seems like no reason.
You can't argue with the quality of their output, but if it switches off before your heroic overtake, that won't matter. Unfortunately that's what happened to me last time at Silverstone. Huge five-second drift through Copse corner to overtake someone. Got to pits to find it had stopped recording a few corners beforehand.