I just moved from VM to Sky.
Picture quality on my old TV is the same. Reliability seems 150% better (no need to reboot every day). Functionality is about on par. Content is better (for us anyway). BB is waaaaaay better (I was on only 2Mb .. now on 8Mb and actually averages around 7Mb).
But the killer for us is cost. We were being screwed for around £82 per month, which for a better package from Sky is costing £52 per month !!
I now VM Retentions offered to match Sky's price but the TV line up and 8Mb BB were the decider for us.