I've been very happy with my Corbin.....until now. (More on that in a minute).
I'm 6'4" 250lb, I bought it new direct from Corbin, it changed the seating position a little but after breaking in for a while (Corbin says about 2,500 miles and I'd say that's about right) it fit very well and 400 mile rides are no problem.
They make the seat for your height/inseam and weight, so if you're bigger/smaller than the guy you buy a used one from, it won't fit. Imagine someone my size buying a jacket from someone 5'6" 145lbs and then complaining about the fit. Same thing.
Now for the "until now" I bought the seat in October of '07, put around 25,000 miles on it and the seam across the seat has split. I e-mailed them two weeks ago and haven't heard back, just tried another direct to service boss. We'll see, I may have to pay for the repair, but I don't mind if it's not outrageous and doesn't take too long. The covering is warranted for one year but the foam and pan are forever so if it's a foam breakdown problem they should cover it.
