I use Kevlar 17 and whatever cheapo synth gut 16 is lying around. I'm not sure if it gives the spin that poly does (there's a debate about whether slippery or coarse strings give more spin...kevlar is pretty coarse). It does, however, last forever, reduce power (so you can lower string tension without launching the ball), and remain playable long after it looks hideously shredded, where as most people will tell you that poly sucks after a few sets of playing.
The key thing about kevlar that most people are stupid about is this:
"Don't use kevlar!!! I strung my pro staff up at 65 lbs with it and it hurt my arm!!"
No kidding stupid ***. You don't need to string up to 65 lbs with Kevlar to have a stiff stringbed. Try 50 or 55 pounds next time, Einstein. I guarantee that your elbow will be sore after hitting with a 65lb kevlar hybrid for 3 hours.