Really unique and low system requirments. Definitely best to use a good mixing engineer to get the most out of it. Pairing it with FORTI and SERTI convolution (add on to the Viennna Suite) can really make it sing.
The original S.I.R. is still free, though high latency. TL Space may still be bundled with Pro Tools and Perfect Space is bunled with Sonar. All the others I have encountered cost extra.
Many manufacturers bundle large collections with their engines, like Waves and Audio Ease. One of the only companies that sells collections of real spaces not tied to a given engine is www.NumericalSound.com, though their last library is different and only works with Vienna Audio Suite.
Must say it is really interestting thing. Sounds react like real deal, but in sound there is something strange. Must test it again a bit later, still a bit drunk or something.
The first test: the violins. The violin sounds seem to be working really well, except bottom notes where sound begins to turn little strange. Machinegun test (short fast notes) works really well. Overall quite impressive.