Specialized mountain bikes are excellent bikes. One thing you have to remember is that each of the manufacturers you listed make a whole range of bikes that are suitable for all types of uses.
It's actually kinda hard to track down an authorized Specialized dealer. They have very few retailers around the US. They are actually regarded as a very exclusive manufacturer and have a great reputation for making high quality frames. Their high performance bikes can cost as much as $7000-$8000. These are full carbon, dual suspension with the highest end components.
Cutting to the chase, their mid-range mountain bike, the Stumpjumper with disc brakes and good components has an MSRP of about $1000.
EDIT: Okay, the guy below caught me. I evidently don't know what I'm talking about. Apparently Specialized now (like nearly every bike manufacturer on the planet) gets their frames from some Asian producer. In terms of widely-available bikes (not some small, boutique road bike manufacturer), most bike frames are produced in Asia. Only some Cannondales and Trek's OCLV road bikes (Madone, etc.) are made in the US. So then the question is, how does the frame perform? Specialized has a reputation for producing great bikes and their frames, even if they do come from Asia, are usually well-made. Giant actually does produce good stuff and they stand by their warranty very well.
All of this origin of the frame stuff doesn't really matter, it's just a game of "gotcha" for cyclists. Point is, they make very good bikes and chances are you'll be happy with one if you get it.