Is Specialized brain something unique or are its variations common? I am looking into getting a full suspension mountain bike. Specialized touts their brain system and I'm wondering if is this some unique technology that they have or are at least more advanced at making than other manufacturers? Or is it that inertial systems like this commonplace and "brain" is a case of good marketing? I'm looking at bikes around 2000 USD (possibly a cheaper Trek), do I need to make extra sure it has a similar system in place or is it a given in this range?
Inertial systems like the brain have been around for several decades, however, Specialized is probably to the first company to do them light (ish), mass produced and marketed.  The real question is not "how good is it?" (is does work), but "do you want/need it as a rider?"

I worked at a Specialized shop for sometime, and opinions on the brain seem to vary.  I know several riders that swear by them as one of the most awesome things ever, but also people didn't feel like it was needed or just couldn't tell the difference.  Personally, I always felt like they added a bit of efficiency, but since I rarely rode trails that benefited from it, and that I didn't MTB race, it wasn't really necessary for me.  

The brains "automagikness" isn't necessarily standard in the price range, however easily adjustable suspension is.  In that range you'll find most suspension has some sort of quick adjustment knob or switch to vary the performance of the shock.  Such systems will likely weigh less, but require rider input.

It should be all about your riding style.  If the trails you frequent don't require suspension adjustment "on the fly", the brain will likely just be extra weight.  If they do, but you don't want to deal with flipping a knob or switch, and you like the way it feels, it may be for you.