I had a seatpost mount rack briefly. You'd be surprised at how quickly the weight adds up when you count the bag you use, and a few things you want to carry. It kept moving around even though it was as tight as it could go. And it felt unstable. I got a Topeak Explorer rack (built for disk brakes) and I love it. I don't notice it when I ride. It can take much more weight than a seatpost mount, and the bag doesn't get in the way of my saddle. YMMV.

Deb

PS If you really want to try a seatpost mount, I have a Bontrager I could send you.