GoPro sent me the Hero4 Session Reviewer's box this week, and I just posted my unboxing video (
Hero4 Session Unboxing - NEW GoPro Hero4 Session! – VidProMom).
I'm not sure I would buy it for $400 if I was in the market for a GoPro, but it does have merits that "regular" GoPro's don't have. It really depends what you (personally) use a GoPro for - if you need the smallest, most versatile camera for your projects, then $400 is well worth it.
It does come with 2 frames, the standard and the "low profile". It comes with the ball joint mount, which is pretty cool all on it's own (although I have a hard time getting it to rotate, but I think it just needs to be warmed up or something).
It is waterproof without a waterproof housing, which is a feature worth noting here as well. To me, that is an obvious (but overlooked) advancement of GoPro's technology. It also has two microphones, both waterproof.
If I had $400 to spend on a new GoPro, it would probably be spent on the Hero4 Silver, because I use my GoPro to capture family stuff. But if I did a lot of snorkeling, surfing, or motocross, the Session might be a good choice.