I have Freesat, and certain programmes are broadcast in HD, ITV and BBC, and the difference in picture quality is superb. Coronation Street on ITV is broadcast in HD, and you feel you are in the Rovers Pub. Some football matches are in HD, and you can see the blades of grass on the pitch. Worth having, but I wouldnt pay Sky a fortune for it.
I bought a Freesat tuner, and coupled it up to my existing dish, and cancelled my Sky subscription.