Any specific model your looking at?
The Diamondback frame, while still being perfectly serviceable, won't be as finished as some of the other higher priced LBS only bikes(Trek, C'dale, Specialized, ETC). The welds won't be as clean, or filled\ground smooth. And the wheels probably won't be as nice(although the wheel on my Specialized Roubaix are pretty crummy, flexy).
I work on a lot of bikes of every level, and Diamond back is usually a middle of the road bike. You could do a lot worse.