I wouldn't go doing drop offs or anything like that with it, otherwise I don't see why you can't give it a go - make sure you check the bike over first though - all the bolts are tight? bike is in good condition etc.

I had a kmart bike when I was back at school (basically a wallmart bike) pretty much cheapest you can get - you know a 26" mens, steel mountainbike with no suspension. I used to ride it up and down any trails I could find in the local hills. It wasn't exactly designed for off road use and it was a 18kg monster but it held up pretty well. Eventually things wore out though mainly because the components were cheap & never replaced.

Based on my experience I'd say give it a go, but be sensible aobut what you ride with it because it can be dangerous. If you take a liking to that type of riding then I'd highly recommend saving up for a good entry level mtb - it will make the riding better all round, and there's noting stopping you using the mtb on the pavement either.

Just a note of caution - if you do decide to try drop offs etc with it, you may find that the wheels will bend or go out of true as they're not designed for that sort of thing - probably not as strong as mtb specific wheels.