Anyone have any experience with the Airhawk inflatable seat cushion? I'm tired of spending hundreds of $$ to modify seats, which sometimes doesn't help, and which you lose when you sell the bike. Seems like I could use the Airhawk on both of my bikes (R1100R and SV650S), and transfer it to future bikes. If it adds to legroom, that's all the better for me. Do they stay put or tend to slide around? Do they help prevent sliding forward on the seat? Do they make it difficult to hang off? Are the small or medium sizes uselessly small? Thanks