Their reverbs are reason enough to get into UAD. The Lexicon, EMT, AMS, AKG Spring, and Ocean Way plugins are all killer. They'll eat your wallet alive, so proceed with caution.
The newer Waves offerings like the Abbey Road plate and H-reverb are excellent, but also munch on your CPU.
One I haven't seen mentioned yet but worth trying is
Acon Verberate. Excels at real-sounding spaces, and is relatively friendly to both your PC and your wallet.
Cheers,
Eddie