Listen to their "Giant Steps": http://www.myspace.com/clustervocal I guess it's one of the most perfect vocal arrangment I've ever listened.
I've seen them Saturday night at a concert, and yesterday they took a masterclass, they talked about their influences and the history of a-cappella music, about how they write their arrangments, about vocal percussion (2 of them are considered to be on the top 5 of greatest italian mouth drummers), and other things...
Finally they took 5 of us and made us sing their version of "Halleluja" (J. Buckely), it was hard, but simply beautifull!
Here you are the video: http://dl.getdropbox.com/u/224179/Halleluya.mov (I'm the one with the USA crew neck on the left, you can see me just at the beginning, but you'll ear me...I sang too much loud for a vocal group...I was the tenor, but I'm a bariton, so these lines were really high for me to sing

Spread Cluster, because they really worth a listen!
Regards