The problem with the ones like the Nashbar that mount against the seatpost is that they can only be so wide without interfering with pedaling. The shape doesn't lend itself to easy packing either.

The Nashbar one in particular (if it's the same one I've got) doesn't seem like it would last too long either. It's not good for much more than tools and tubes as others have said. A small saddle bag works just as well and doesn't block access to your water bottle cage.