Get one! They aren't cheap as was mentioned but there's nothing like flying down a track through trees, over rocks and getting a little sideways on some loose dirt. I used to do a lot of downhill racing when I was younger but ended up selling my bike so I could buy a car when I was 17. It's not that I regret it but I seriously miss it. You get what you pay for (much more so than road bikes IMO) so if I was you i'd decide if I really wanted one then save up for something really good. And don't forget you'll go through parts quicker because of the abuse you'll undoubtedly give it.
Picture for old times sake: