The sounds are very good, I bought this specifically to get great orchestral string sounds. Surprisingly when I found what I wanted in one of the libraries, I also found a sound in Alchemy (as part of Final Cut Pro X) that was indistinguishable from it. I guess it is a sign of how far synthetic sounds have come.
I purchased this virtual instrument last year and I have to say it's one of the best decisions I've made. The sheer amount of sounds and the sound quality are well worth the investment. From choirs, to strings and church organs, M-Tron Pro has it all. You can get those Steven Wilson, King Crimson mellotron sounds with ease. A must-have.