I was on o2 for 3-4 years before being transferred over to Sky last month. Since transferring, I've had nothing but problems. Online gaming pings have gone from 20 -> 50, download speeds have fallen 20-30%, my connection frequently hangs (Youtube is near unusable), and I now have issues loading websites from time to time (I had to refresh Google 3 times before it loaded earlier). To make things worse, Sky is charging me £15.50 a month while o2 was only charging £7.50.
Thankfully, BT is installing fibre in my area so I should be able to jump over to plus.net sometime soon. I can't wait.