Looks like the rear derailleur is broken. Any frame damage or just the derailleur? Overall, it is a pretty cheap IMO, but depending on what you plan to use it for and what your budget is, it is usable and can most likely be fixed for much cheaper than a new bike could be gotten. If you plan to do much serious riding, I doubt it will hold up for long. If it is 7 speed and that is all that is broken, derailleurs can be had for under $15. My advice is that if you plan on doing actual mountain biking, fix it then sell it and buy a better bike if you can afford it.