mtbr.com has a pretty extensive list of reviews for mountain bikes.
I personally love Gary Fisher Bikes, I have an 08 HiFi Deluxe and was well worth its price tag. That bike is a pretty vanilla entry level bike, not to say it isn't a good bike but, you will notice a difference between that bike and some lead sled from wally mart. It will be a much more enjoyable ride to say the least. It'll give you a chance to evaluate if mtb'ing is something you want to pursue.
The important thing is make sure you talk to some one who is knows how to fit a bike! if you get a cheap bike it may be due to the fact it is a small frame and they can't get rid of it. Too big or small will make for a miserable ride.
I live in roadie central and had a hard time getting fitted for a mtb. ended up driving up to atlanta from Melbourne Fl to get the price I wanted and fitted properly.
If you can be patient, between october and january, bike shops will start cutting prices on that years models to make room for next years.
Also, people have a pref of one brand over another, GF, Trek, Kona, Gt, etc etc.. they are all great bikes if you get it from a reputable dealer. The best bike is the one you happen to be saddled on so go out and get an epic ride on

edit- Crap! how could I forget Specialized!