During 5-mans in the beta while tanking, I saw an HP boost into the 26-27K range from a health pool of 19-20K. That's certainly nothing to sneeze at and it's situationally useful, although it'll probably find better and more consistent application under the same circumstances in which Last Stand's typically popped, i.e. progression raid content and/or "Oh S**T!" moments. The latter is particularly welcome as Druids have often complained about bear form's worrying lack of options in the event of an emergency. As many Warriors will tell you, proper use of the ability involves letting your healers know both when you've popped the ability and when it ends. Mods like OptiTaunt can relay this information automatically, but you can also macro it if need be.
Apart from that, Survival Instincts will certainly see use by cats in PvP, although I'm more curious as to whether it'll see extensive use by restos or panzerkin. In Wrath, Survival Instincts will occupy the 11-point feral talent slot vacated by Feral Charge (now a 21-point talent), but I think all of us can agree that it's no replacement for Feral Charge's snare and interrupt. With that said, if you're still willing to dump 21 points into feral for PvP purposes, it doesn't make any sense not to get SI, but I'm a little leery over predicting potential PvP specs at the moment with so many class talents and abilities in the beta being reworked.
As far as I'm aware, Survival Instincts will stack with health-increasing trinkets like Shadowmoon Insignia and Battlemaster's Cruelty, and there won't be anything to prevent you from popping SI, a trinket, Frenzied Regeneration, and Barkskin all at once for those ugly, boss-at-1%-and-most-of-the-raid's-dead moments. For tanking purposes you're probably better off saving Barkskin for the moment that SI wears off and your healers are making the transition, but all four of those (or some combination thereof) could really make life hell for a PvP opponent.