My VirginMedia connection is fine...
I think that the lesson that we have to learn from all of the contentious feedback here is that VirginMedia, like ALL other ISPs vary in their performance from region to region and line to line and also on what you plan to do with the line.
I know that I get the full 20Mbps speed from my VirginMedia line, I get that in speed tests and also through Torrent downloads, which actually peak out at 2.3MB/s when downloaing things such as xubuntu images for which there are many sources available.
My boss is also on VirginMedia, and also on the nominally 20Mbps connection package, but doesn't get anything like the speeds that I do. One of my colleagues also switched to VirginMedia on my recommendation and he does also get the full 20Mbps speeds (phew!).
I can't comment on secret limiting of my line with VPN or other things like that, I don't use VPN, but I just don't get problems with all the usual stuff like email, web, streaming media, iPlayer, FTP, Torrents and so on. Yes, I have hit their download caps from time to time, and to be honest, 5Mbps for a few hours isn't actually that limiting.
When I was on ADSL to the same property, I never got close to 5Mbps speeds on my nominally 8Mbps connection, BUT as I say, I am sure that there are people out there on 8Mbps ADSL lines that get 8Mbps service, it all depends on where you are and the quality of your line, the distance from the exchange (or "green box in the street"), and the number of other people you're sharing your pipe with.