I got a Spongy Wonder for free from a 1973 Schwinn World Voyageur that I found out for the trash. I called the police about the bike, who called the owner, and he told them he put it beside the dumpster because he didn't have any interest in fixing it. Two new tires and a derailleur adjustment later, I had a bike that rode like new.

When I sold the bike, the guy told to me to keep the Spongy Wonder because he was just going to replace it anyway. I used it on that bike (a road bike) and a mountain bike (mostly road and bike path riding). My impression of it is that it feels like you're sitting on top of the bike like one would sit on a chair, versus riding in a saddle like being on a horse. It's comfortable, definitely, but control does suffer slightly (as stated above).

If you've been off from biking for a while, and just started riding again, it won't give you the pain other seats will give you. It takes a little getting used to, and there's an underlying fear that your most important parts will bounce off the top of the seatpost where it attaches. However, this fear has been proven unfounded over miles of riding (though I rarely use the seat anymore). I would not use it for biking over rough terrain, though.

Neat seat, if you find that design necessary for whatever reason.