im on Be and it is a spot on service!
For peeps on be, phone up, tell them you want to leave and request your MAC key... they will offer you reduced monthly charges. I am on the unlimited package and was paying £22 a month. After requesting to leave, the monthly charge went down to £12.
If you are a new customer, before signing up, ask for a discount, dont be shy about it, just ask. They will offer 3 months free...
hope this helps
Richard