A well-made , properly fitting sheepskin seat cover makes a world of difference. Had one on my BM GS ... they are great if you're travelling. Sheepy Hollow made the one i had.
After having sheepskin covers on other bikes they do have their benefits but also the downside. They are comfy but you have to put in a bit of time now and then to fluff them up a bit or they just get flat and the comfort factor goes out of them. Secondly as stated by TC they must be a good fit or they can damage the original seat cover as friction may cause wear on the edges where it folds to the underside of the seat. Hope this helps you decide.
In my opinion the sheepskin, for the money, is a good investment in butt comfort. I bought my sheepskin from Alaska Leather. They have very good pricing and the customer service is good as well. The covers I bought lay on the seat with straps to hook under but I chose to cut the straps off and just toss them in the saddle bags whenever I stop. They also come in a variety of colors and from short shear to a long shag like look. Here is the link - <Alaska Leather> Home of the Sheepskin Buttpad - Motorcycle Seat Covers - Alaska Leather Home Page
A little over 16,000 in 2 years, but it was one of 4-6 bikes I owned during that time. I've done a few 500 mile days, many 200-400 mile days, but as others have said, seats are a very subjective thing. I sold a Harley Dynaglide because it killed my back after 60 miles, and I couldn't say why, so I refused to throw money at it trying to make it comfortable. I've done 500 mile days on my stock 78 CB750K in complete comfort, it's almost the equal of the Rocket for comfort, surprisingly.
The seat came on the bike, the only one I've had on it.