My only experience with Diamondback was with a Response last year. I won't comment on the bike because I only had it for 3 days. After going round and round with their customer service and then build department, I took it back. Not only were they not helpful, they were rude. Two lessons I learned in that experience.. 1) Don't buy bikes anywhere but from a LBS (with the possible exception of REI who actually offers great service. and 2) Don't buy anything from Raleigh Bikes (who is the maker of Diamondback).
Is it possible that my experience is the exception rather than the rule? Certainly. But money is too hard to come by to give it to a company that shows no appreciation or obligation of support for it.
I ended up buying a Trek from a LBS and have gotten outstanding support ever since. I have no idea if the DB is a better/worse bike than the Trek I ended up with, but I know that the experience I got instead made not knowing worth it.
I built up a few-year-old Response frame for a friend just recently, and I think it's a decent frame. MTBr reviews are good and seem to indicate the frame is quite strong...it certainly looked burly enough to me. The bike has been ridden a few times with no problems so far.
In regards to Gorsar's comment - A lot of shops around here carry Diamondback. I agree with him that you are much better off buying any bike from a shop as opposed to online, or X-mart, etc.