Personally I've hated Virgin since I joined with them, but I think it's just my complaint with the way ISPs are going right now. Virgin have absolutely fantastic speeds off-peak, but I frequently have lacklustre speeds during weekday nights, from about 4PM until about 1 in the morning. Partially it's down to the fact they have a wishy-washy download limit, and say that they'll clip your internet if you 'use it too much', and partially because of the huge amount of people on Virgin now. If I wanted to watch a TV show on iPlayer right now, my connection wouldn't be good enough, though browsing is fine at this speed. The odd thing is (slight tangent here), some days my download speeds will be fantastic, but browsing will be ridiculously slow, any ideas why here?
Anyway, if you don't need to do loads of downloading, go for Virgin. If you're really download-centric just go for Talk Talk, despite this talk of them moving you around they still offer the best by-the-numbers internet service.