I have about 500 miles or so on the SKWOOSH ICS Gel Seat pad. Alot of my riding is on I-80 from Roseville CA to San Francisco for my workweek. (115 miles one way, there and back each week)
I have to give it a solid B+ for the overall evaluation.
I put it a little bit forward of where most would think to put it, and my tailbone can sit in between the gel pad and the pillion. This gives me the most comfort, by putting most my weight on the pad resting on the back of my upper thighs, (below the glutes) Some butt support falls against the pillion, but the real improvement comes from getting the tailbones off the original seat surface. I have had NO NUMBNESS OR CRAMPING since using this. My knees have even done better due to a better angle, but I still plan on getting UFP foot peg brackets nonetheless.
The seat comes with a strong fabric elastic band which holds the pad in place even when the seat is lifted for storing. It is very easy to take off and secure if in a place where I am concerned about it.
The underside is made from a slip resistant, waterproofed rubberized fabric that does a good job and won't shift around once it's in place. In this photo, the underside is in the middle, and the sitting surface is on the right.
And here is one of my favorite parts,
About the only downsides to this, is the cost of the unit, (+/- $70) and that it lifts me about 3/4" (18mm) which is a fair bit since I have short legs.

If I lost this, I would be at the motorcycle shop the next morning to buy another. 8)