Well I can't comment about 50Mbit as I'm still on 20Mbit but since it's dropped in price I seriously am considering it.
Overall I'd say they're pretty good, not had any major problems with them, most of the time I get the full 20Mbit, occasionally it can slow down a bit but it doesn't seem to be too often, and even then I get about 16 to 17Mbit.
I'm not sure too if they're still doing it but when I signed up I got the M TV package (basically pretty much what you get on Freeview, plus on-demand things like iPlayer, ITV Player and 4oD on your TV) for free. Although we use Freeview, I figured since the box was free it was worth it (the original idea was to put the box in the bedroom, nearly 3 years on it still hasn't happened

).
If you already have the cable box and phone connection to your property you might be able to go for the self install option (means you get the kit quicker and you don't have to wait for an engineer to come round, although in our despite choosing self install the engineer came around the day after placing the order and connected our phone line to the cabinet). If you go for the engineer install option then you might have to wait a week or two for them to come round.
I'd also say maybe keep the Be broadband and phone line until you've got the Virgin broadband installed just in case there are any unforeseen problems or delays (probably won't be a problem but it means you'll at least have a connection just in case).
Rob