I've used gumtwo seat covers on both a 550 I had and my current F2 with great results, the one on my F2 has been on for 4 years now and still looks great. The only problem that I'm aware of with gumtwo seat covers is that he had issues with the strap that runs across the top, what ever vinyl he was using although it worked great as a seat cover it wasn't suitable as a strap so he stopped including the strap with the cover. That's been a while back so I don't know if he's corrected that or not, on my 550 i just reused the original strap and nobody noticed. Still a hard price to beat if you're not worried about keeping the bike 100% original.
EDIT
Another thing to keep in mind is that if you have a seat that he doesn't currently make a cover for all you have to do is ask, if your old seat cover has enough "meat" left to it for him to make a pattern from then he can probably make another one for you. The reason I know this is that for a brief time I had an aftermarket king/queen seat on my F2 that was starting to get torn up but being it was aftermarket he didn't have a cover in stock for it so i simply asked if him if he could make another one if I sent him my old seat cover and he said yes. I never got around to it because as soon as I got my 1100 interstate the king/queen seat on the F2 was removed and the original seat (with his cover) went back on.