Survival Instincts is one of the surviving (har!) aspects of the now-defunct 51-point Berserk talent in the feral tree (the other is the new version of Berserk, which we discussed here). It's fairly straightforward; for 11 points in the feral tree and a 5-minute cooldown, you get 30% of your maximum health in bear or cat form for 20 seconds. Essentially it's Last Stand for Druids, albeit a Last Stand with a significantly shorter cooldown.
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.