Sky are OK and as a new customer you should be able to get a decent deal depending on what you want. I would call them rather than buy online and negotiate a price you are comfortable with. As per any provider you will have to renegotiate your contract every year. They are very good at sneakily increasing your charges so keep a close eye on your monthly bill which you can do online.
I did mine recently and actually went to the extent of cancelling altogether. Within a week I was getting phone calls and mail shots with much better offers compared to what they were offering me previously. I did consider Virgin but just can't get on with their Tivo boxes. I much prefer the Sky Interface.