Very good service, I used to have the 50mbit service which was fantastic. Moved a couple of streets away and no VM service, and back to [censored] ADSL.
Went from 50meg which was flawless pretty much all of the time to ADSL2+ which syncs at 10meg if I'm lucky. Intermittent line fault that took 3 months of o2 and BT passing back to each other.
For me the big pros of VM are:
1) Faster all round. Sure, you will get congestion in some areas but I would be surprised to see 50meg fall to less than 10meg on a regular basis, which is probably the average ADSL2+ sync speed in urban areas.
2) Ownership of problems -- if something is broken, VM can't blame openreach. Generally makes things a lot quicker to get solved.