The tipping point for me, for purchasing Omnisphere was when I heard the demo of some of the human voices and the adagio Fairissimo by Jordan Rudess in the NAMM video. Just had to have it and I am not disappointed. Omnisphere is superb although I am only just getting into it. However I hope that more classical voices will be added in the future as it is frustrating that I cannot play the Soprano solo with the tenor solo as the Soprano sings Oh whilst the tenor is limited to Ah. Both are truly amazing but if they could sing the same vowels it would be much better. Also I don't want to be greedy but a classical Bass and Alto soloist so would be the icing on the cake.
Must get back to my Adagio strings, they sound like heaven.
But remember those are taken from their old sample library. Not sure, but isn't everything useable in a keyboard context already in there (I don't know if there are more samples that are waiting to go in- I wouldn't think so).