Been on Sky for about 2 years. First on ADSL and then on FTTC.
ADSL. The speed dropped from 9M (LLU provider) to 6.5M SKY, but that was expected due to how SKY run their ADSL type services. Never had a problem with performance of the line or any download problem. As FTTC came to my area, I decided to move to that type of service.
FTTC. Moved to this service in Nov 2012. Line running at 40/10 (service speed I bought). Not had any performance problems with this service at all.
I understand some small number of specific exchanges have had a performance problem, as SKY have more customers using the service vs. throughout. Listening to the message boards this looks to be a short term situation while additional capacity is ordered.
So finally, if the SKY service is in your price band, then go for it.