I've got 5 of the things - 2 3+ blacks, 2 HD2s and an HD.
I've been using them for years for my adventure sports - paragliding, paramotoring, sky diving, scuba diving, snorkelling, motorcycling, mountain biking, etc.
There really is nothing to touch the quality of the optics on the 3+ (and now probably the 4) - pin sharp edge to edge at 170 degrees is awesome. Plus protune records in insanely high bit rate (40+) giving amazing artifact free footage.
The 3+ is a pro tool, the 4 will be too. TV programmes are full of gopro footage these days, from TopGear, to movies. It's the defacto POV.
I can shoot incredibly 3d footage with the 2 3+ next to each other (I make a wee 3d mount you can download from thingiverse if you want it), and mounted on a gimbal (I'm stu of stubilizer 'fame') the results take some beating.
here's Skye from the air:
Shooting at 2.7K in protune allows post edit cropping and still leaving crisp 1080p footage too.
Battery life is a bit pants (I use external lipo solution I built), but the 4 seems to have bigger batteries.
I am tempted by 4k - (both my macs are 3k screens) but for now, I'm gonna stick with my 3+s I think.
I think the budget one is the killer though - that's only a slight premium over the far inferior quality chinese jobs (SJ4000, etc) - with the great optics and decent sensor/ambrella chip I think they might have really chopped off the cheap chinese market.
I'm off to Menorca next week - and will be packing the gopro as always - this time, for some snorkelling footage (red filter on lens). :-)