Mtn bikes have 4 primary and distinct distribution channels:
1. Cheap-skates---discount retailers like walmart=cheap bikes that will last 1-2 years with crappy performance, think "Huffy"
2. Wanna-bes----Sports retailers like Dick's or Sports Authority=cheap/moderate bikes that will last 3-4 years
3. Pro Shops----Specialty Bike Shops=You get what you pay for here...brands usually vary from one shop to another. Trek, Giant, Raleigh, Specialized, Gary Fisher, LightSpeed, Cannondale, etc...
4. Web Shops---The New Kid on the Block....huge YMMV. You won't get the service and support for your bike you'd get through a bike shop.
Keys to buying the bike for you. Figure out what kind of bike you want to get and how you'll be riding. If you're doing off road riding, I recommend getting middle of the road components. No need in spending extra money on a bike with racing parts if you're not going to be racing. It will be a waste of money.
Make sure the shop you go to doesn't try to force you to buy a bike you don't need. Some will try to sell you what's expensive to get more commission. See if you can find a shop that doesn't pay their employees commission and has a strong service dept.
Component levels for Shimano I would recommend are LX, XT, or XTR....from those three, I would try to get a bike that is all comprised of LX or XT components. This would include your deraileurs, brakes, pedals, shifters, etc. Deore makes a lot of their stuff and is a little lower on the quality level than LX, but it is acceptable. Just make sure you get quality parts...they make or break the bike. I recommend a Specialized branded bike....they're GREAT bikes.
I personally have a Raleigh M80. It's served me well for 5 years now and is probably about the stock quality level you should consider. Looking at. Review the prices and features of this bike and see if you can find its match in other brands.
Canondales are manufactured in PA if I'm not mistaken. It is an East Coast favorite and will be overpriced accordingly. No matter what anyone says, they're NOT that much better (if at all) than any other bike. It's all hype that's going to start from the commisssion pushers I was telling you about earlier. Good luck on finding the right bike for you. Look to spend around $500-600 but don't pay too much more for an LX quality bike.