I recently picked up a gel insert and a piece of memory foam and did a hatchet job on my seat getting them in
But once I got the foam cover thinger on and the stock seat cover back on it looked pretty much exactly the way it did before (slight rise as I didn't get it totally flush and I mean very slight) anyway long story short, after a couple hours of riding my butt would generally get fairly sore, now after a couple hours riding it feels more or less the same as when I jumped on the bike.
It was pretty easy to do, just a hours work or so, little messy cutting the foam on the seat, but for $50 off an ebay store it has made one hell of a difference and I'm glad I did it.