I'm with Be (well actually O2, but they resell Be's service) and have only positive things to say. It's been reliable, and I live in an area where most ADSL services are hugely over subscribed. I've also got a friend a few towns away using O2 and it's fine for him too.
The router you get is completely rubbish, though, so you'll want to either bin it or plug another one directly into the back and use that to connect instead. Well I say 'bin it', you're not actually allowed to do that as Be want it back at the end of your contract, but you get the idea.
I'm not sure what speed you're getting with Virgin, but do be aware that - particularly if you're an ADSL
virgin newbie - your speed does depend on your distance from your phone exchange. Happily
BT have a tool on their website that lets you check what you're likely to get, so give that a go before you sign up.