If your TV is HD, then get it.
The pictures better by a long way.. and if you watch a lot of Premiership football etc, then it'll make a big difference.
Films looks awesome, series looks awesome (Hereos, Prison Break, etc).
I love it.. but then, my favourite stuff is all in HD.. so..
The way it works is that HD is £10 a month extra, and then any channels you have normally, if they have a HD version, you get them. So if you have Movies and Sport, you'll get the Movies and Sport HD channels.. if you just have Sport, then you'll get Sports HD but not Movies etc.