I dont think its worth the tenner extra, I was an early adopter to sky HD when the first boxes came out, I ditched it when I moved in June. It is good, it is better, but like TV size, it didnt really affect the pleasure I got when watching something, it just becomes "the norm" and makes you less likely to watch something in SD.
I packed up sky completely when i moved and just watch regular TV, I have freeview HD now, but I rarely bother with it, the only time id switch it on if if there is football on in HD because ITV's coverage is normal is a really shockingly low bit rate.