I put the Creature Comfort seat in my WoodDuck 12, and I'm quite happy with it.

+1 to what Peter says. And while they may seem a bit pricey at $128, I've been using the same seat since 2004 (and it's still going strong) so you get to divide the price by at least 13 which brings it to below $10/year (less than 3 cents per day). That's a very reasonable price to pay for no back pain. The velcro on the bottom and the snaps I put on the straps also let me move it between boats, so one seat is all I need.

Laszlo

I use a SweetCheeks 200 after trying various options to prevent leg pain and my feet going numb. 

http://store.jacksonkayak.com/sweet-cheeks-200-black/

It has proved excellent and I can comfortably spend a few hours in the kayak (Chesapeake 17LT) whereas before 40 minutes was about my maximum. I was considering a Creature Comfort seat but it was a bit pricey for me and with shipping charges and tax to pay (I live in Turkey) it was going to be too pricey. The SweetCheeks is cheaper, moulds to fit your bum, can be moved between boats and I could buy one in the UK and bring it here without the added costs of the Creature Comfort. 

I'm probably building a Night Heron S&G this winter and will probably buy another SC200 for that - a friend will often come along using the Ches..

   I go with NSI (North Shore Industries) adjustable touring back band in all my boats. They are absolutley the most comfortable back band I have ever used, but that said, no back band replaces a normal chair seating. They're 9" high which gives lots of support for hours in the cockpit, but not as great if you're into Greenland rolling. 

   Creature comfort with discovery thigh support works for my shear water .making your own  out of foam  is maybe even a better option.the problem is most seats sold are for sit on tops that have the seat area  already contured while clc boat are basicly flat. You need a way to raise the area under you thighs up to spread the weight .you can do a better job of that by carving your own.