I've used Altiverb (a Mac convolution plug-in) since it first came out. It is AMAZING at doing natural reverberation and ambiance. Not just big environments, but also smaller ones. I rarely use drum samples on a production without putting them all in a convolution drum room, to get some realistic 'space' around the usually too dry sampled sounds. It is amazing how this simple step livens up a sequenced track and makes it far more 'real'....

Plus, any orchestral work, string sessions, whatever, you can put in a proper sampled hall, Concertgebouw, Carnegie Hall, La Scala, etc.. Compare any of these to your 'Hall' preset on your arranger's reverb, and you start to realize just how MUCH you are missing!

Highly recommended.