What's great about it is that since it's a square shape, this means it is fully functioning in all angles. Therefore, if you want to side mount it, you don't need to go out and buy any extra accessories, which really add up, particularly if you're buying the GoPro brand. You can also use your phone charger to charge it up - really convenient - and it allows you to use your phone very easily as a remote so you don't have to go out and buy the £65 GoPro remote. HOWEVER. Personally, what I find is quite inconvenient, is that there is no real screen display on the camera itself. The small LCD screen shows you your settings whilst it's recording, however to change any of these settings you need your phone paired with the GoPro App, which you obviously don't want to take with you when you're going out on open water. The other downside is that the Session is only waterproof up to 10m, whereas the Hero4 can get down to 30m. So it's not going to let you go scuba diving very deep.
That said, the Session it is so easy to use, so much fun and for those reasons, some of the best money I've ever spent. I've put together a video of some of the footage we got from sailing our Dart 18 and also some very tame(!) bodyboarding, which features a large handful of my family and friends! Don't forget to bump up the quality to 1080 HD!