I flew UC from LGW to MIA last Summer (out in July, returning in August) and had a really good flight.
Virgin used a "Club America" lounge at MIA which isn't anything special: comfortable chairs, free self-service drinks, peanuts and mini pretzels, but that's about it.
You can find a full trip report at
http://www.flyertalk.com/forum/Forum81/HTML/002066.html . I flew on a Continental ticket (an awful lot cheaper), but your experience should not be much different (apart from you will use the Virgin Arrivals facility and not the Continental facility).
In summary though, I really enoyed the trip (the crew were great). I am sure that you will have a great trip too.
Boo
(trying to work out if she can afford to fly UC again this summer... Flying UC spoils you :-( )