I hate to disagree with P2P about anything (because he's very knowledgeable and helpful) but I think that Linear FOV is awesome, and not at all a gimmick. I'd suggest at least TRYING the Linear FOV feature of version 5 on several flights yourself, and comparing its result to the standard GoPro Fisheye FOV's. I personally find that the benefit of having Linear FOV far outweighs the loss of auto-turn-on. You can easily revert to version 3 at any time if you want to. In any case, it costs nothing to try both, and you can upgrade & downgrade any time.
Unfortunately, nobody to date has posted a critical side-by-side comparison between shots taken with a linear replacement lens versus the GoPro Linear FOV setting. People here have claimed that the latter MUST be fuzzier than the former, which makes perfect sense theoretically, but I'd like to SEE exactly HOW MUCH fuzzier it is, in concrete examples. Maybe nobody here has both a modded and unmodded GoPro?