I cut out a part of my stock seat and glued a Gel insert into it 3 months after I got mine in 2010. Then I put a sheepskin over it. That worked pretty well until I got a Seat Concepts seat last month. That is even better, but I still use the sheepskin. It lets air circulate under your rear and provides extra cushioning. Out here in the dry high desert I can get away with it, but it would not be so good in areas that get a lot of rain.
I cut the sheepskin large and tuck the edges under the seat when installing, so although I can get it off in a hurry it's not so easy to get back on and stay put. I carry a plastic garbage bag to store it in until I get home and pull the seat partway off. To me, it is worth the effort because it is so comfortable. I can also use that bag as a temporary cover for just a short shower, which about all I ever have to worry about on local rides. Get a natural color if you are going to try it so it doesn't get hot in the sun.
BTW, my gel insert seat foam and original cover are FS in the classifieds for a measly $25.