Experimenting & Skill Development Essential
I've only been an mtb'er about 3 months -- so take what I say with that in mind.
I got a great deal on a hardtail - $250 for a new 1999 Giant Yukon which had been sitting around in a friend's bike shop for 6 years and he just wanted to get rid of it.
A month later I got another great deal from same friend. He had his own 1998 Gary Fisher Level Betty L0 which he only rode several times - I got this dualie for $300.
Now here I am 3 months later after riding both of these bikes quite a bit on trails, reading mtb books, participating in this forum, reading Mountain Bike Magazine online, experimenting with equipment (tires / tire pressures / bars / seat height / pedals), experimenting with techniques for climbing, downhills, mud, logs, and, oh yes, screwing up a lot.
There's lots of good advice in these resources, but you need to get out there and try stuff and develop skills. Hey, a $6,000 mtb would be a waste on me now.
What I'm learning is not just skills, but also what kinds of mountain biking I prefer. So when I get a better handle on my preferences and better skills in a year or so, then maybe I can get a bike that is "better" for me. But for now, my $250 and $300 deals surpass my $200 skills.