I rode a Flyer on my Karate Monkey. Nice saddle, but heavy. It was very comfortable riding off-road -- noticable flex in the springs, but not too springy. The springs were creaky at times. I did some good rides on it (Downieville, Bootleg Canyon, Sun Valley, HITG & Mr. Toads in Tahoe) and never felt that there was any real downside other than the weight. No problems getting behind it for steep downhills etc.
ON and SS, I do most of my climbing out of the saddle, and on a hard-tail I hover for most of the downhill, so I will probably go with something lighter in the future. For fixed I ride on the saddle a lot more, so that's where the Flyer will likely end up.