I have NTL, Five is not on it but from my time living in the UK, you're not missing much
Benefits of NTL is that installation is easy and doesn't take long, you dont have a satalite dish(which may be an issue depending on where you live, some appartment blocks don't allow them and I read in a local newspaper in Carlow of a new housing estate that couldn't have them as the walls couldn't support them!!!)
Downsize of NTL is that their customer service is appaling, not the staffs fault, just idiotic policies and not enough people. I've heard lots of stories as well of installers not turning up. Picture can go sometimes as well, and can be a bit iffy from time to time
never had SKY so can't comment