Depends on what you're looking for...
It really depends on what you're looking for. If you're looking for an all mountain bike, I might shy away from them a little bit, but if you're on a tight budget, it might be a good option.
My friend has a Diamondback from... 2001 I think. Its an ok bike. Its a step up from the department store bikes, definitely, but its not up to par with alot of bikes and he's looking to upgrade it (though I tell him itmight be cheaper to buy a new bike). I ride pretty hard and he has a tough time keeping up with me, and I think alot of it is because of our bike difference; mine being a 2002 Enduro and his being the 2001 hardtail diamondback. He doesn't take great care of it either, so that might be a factor.
What bike in particular are you looking at (if any)? Check out the reviews of them on this site. Good luck!