Many bikes in this price range have similar frames and components. The components won't last as long as on a better bike, but will be good enough to get you started.
After that you pretty much get what you pay for in improved components. Its like buying a computer - the more you spend the better components you get but the higher you go, the less gain you are likely to notice (unless you are really pushing limits).
If you are keener to commit and spend a bit more trail centres should be able to rent you a trail bike.
Look for end of the season sales, or better yet, know someone that works for a supplier or store that can get you a decent discount. Mountain bikes are insanely expensive, but for local stores there mostly isn't a big profit margin (at least not here, Norway). The manufacturers themselves probably have a lot better profit margins, but I do not know enough to say anything about it. At this point, any "known-brand" mountain bike over or around $2000 new is a really solid bike that will work great and last a long time if taken care of properly. Above that you start to see carbon frames, higher end gruppos and mostly weight savings.
For example a bike with a Shimano SLX groupset is basically as good as one with the XT groupset (one step up), the XTR groupset is lighter and costs way more.
the Shimano SLX groupset is comparable to SRAM's X9 groupset in performance and price.
Getting into the sport I would go for a hardtail (no rear suspension) first. They are a lot cheaper, and you will get more bike for the money. Spend $2000 on a hardtail and you have a really great and light bike with a near top-end groupset, but if you spend $2000 on a full-suspension bike you will get less bike for the money, and a hardtail is good for beginners anyway. When you do not have rear suspension you have to learn how to keep your weight on the bike and how to pick the best lines down, and a full-suspension bike is a lot more forgiving that way. I can ride down stuff on my FS bike I'd never dream of doing on my hardtail.
Do your research and watch for deals or used bikes. If you are comfortable working on or learning to work on bikes you can buy online otherwise the support from a local shop will be worth more then what you save.
mtbr.com is great for reviews and has a deals forum and page. bikeradar, pinkbike and some other sites that will crop up in google are great for reviews too.
For reference I bought a new bmc trailfox tf02 with slx components for 1400 a few years ago and a friend bought a well used but well maintained specialized enduro for 900 last year...i'd say these prices are about as cheap as you should expect to find a good full suspension bike though hardtails can be had cheaper.
There are tons of cheap but not great bikes out there. A good rule of thumb is to not get anything that doesn't have disk brakes and ideally you want something with Shimano hydraulic disk brakes as they are great and easy to maintain. Cheap full suspension is to be approached with extreme caution as it will be heavy and bouncy when you try to pedal it.
I bought my first serious bike to a friend for less than 300 dollars.
It was a "first class" bike, but 10 years old, so it had not disk brakes, but good old generation brakes.
Simple hardtails and flat pedals are much better to learn MTB and much cheaper. In fact, if you can control your bike in the air with flat pedals a new world will open to you.
You will be able to do bunnies, Bunnie hops and other tricks that are more important than spending money on a bike.
If you know how to jump 1 meter height with your bike, you will run circles with a 300 dollars bike around anyone who does not in a 5K bike.
If you are a pro, then sure, go for it, but odds are that if you are asking you are not.
Ebay has lots of good deals. You need to research things like the difference between shimano xtr and shimano altus or sram "x" vs sram "y". I got a great 2009 shimano xtr light weight mountain bike for my son. Note 29 inch wheels (29ers) are all the rage now. This means 26 inch mountain bikes are going way down in value but are still great bikes. Lots of good info on youtube as well.
If you have a friend that knows mountain biking well, then going used is a good approach. Pinkbike.com is a good place to start for people in the USA/Canada. REI is also a good place for reasonably priced, new bikes.
