I have used them, great service. You will typically pay a premium for seats, but at least you can get good seats. I would rather pay more for good seats than have crappy seats.
Had trouble once with tickets bought through Stubhub, problem was with the seller, and Stubhub came through great, rectifying the problem and following up with me. They showed real concern and I ended up very happy with the way they handled it. I would not hesitate for a second buying tickets through them again.