While I was with them, I had 3 gripes:
They wouldn't transfer the MAC address for the 2Mbit cable modem to my account for a 10Mbit service, so I ended up with both 2Mbit and 10Mbit cable modems. Virgin simply didn't want to know about the 2Mbit one, so it's sitting in a box in my room in a non-cable town until I move.
They sent a letter claiming that a file was downloaded from the IP address supposedly registered to our account. I passed it to the flatmates and they filed it under "bin".
When they get their billing wrong, you will never ever get the same bill twice or the one you originally signed up to (VAT change December 2008 meant my bill were interestingly adjusted for VAT credit month to month as they played catchup), but boy do they put you through the wringer when gods forbid you miss a direct debit or card payment! My word, took 2 members on here to sort my problems out with cancelling a service and stopping being charged for things I wasn't using!
Other than that, the service was fine. Pings solid, transparent traffic management, and unlimited downloads as advertised. Gaming on the 2Mbit modem was fine! The flatmates never complained about the 10Mbit service we shared.