I've used their amps, subs and components. It depends on which series of product you are looking at (budget) and what you expect.
Their subs are dirt cheap and are nearly indestructible. There are cheaper subs that perform just as well but I've never had a Diamond subwoofer fail or know anyone thats killed one. My last car had a Kicker XS100 running 1 ohm bridged mono on a pair of 12s (probably a realistic 1400 watts) and never had a problem.
Their component sets perform on par with the equivalent speakers from Quart, maybe slightly below on the top of the line stuff. Don't kid yourself though, they can't hang with the top notch stuff from Morel, Dynaudio, Focal or Rainbow.
Their amps are all accurately rated and I wouldn't recommend them for anyone trying to compete for SPL. They are safe and efficient but regulated power supplies and accurate ratings aren't real popular. You have to cheat to win and Diamond doesn't builder cheater amps. Their D7 amplifiers are VERY nice but they are in the same price range as Zapco stuff.