I thought I'd join the lonely party . . .
I've applied to:
Columbia
Cornell
CUNY Graduate Center
NYU GSAS
Princeton
Yale
I did my BM at Cincinnati and MM at Illinois. My GRE scores were more or less average. I'm sure the role they fill varies from institution to institution. When I visited Yale Martin Bresnick said they use them more or less as tiebreakers between equally qualified candidates, and I would assume that's often the case.
I tried to send a varied portfolio of recent stuff. Some schools asked for 2 pieces, some 3, and some allowed 4. I sent out a large chamber ensemble piece (18 players), an electric sextet (cl, t sax, perc, e gtr, bass gtr, acc), a marimba solo, and a clarinet and piano piece. I've heard that schools like to see large ensemble pieces, but who knows. As a friend of mine put it, it's probably better to send good pieces than good instrumentations.
The process is certainly daunting, especially with such low acceptance rates! Best of luck to everyone here . . . I'm on pins and needles.