If you go for the "up to 40mb" package and you are an existing Vodafone customer then you get £5/month off.
No discounts on the "up to 80mb" service.
As for Vodafone as a provider, for me it has been less than plain sailing.
4 days late getting connected due to "an issue stopping new customers being authenticated on to the network".
A connection that started at 80mb and then dropped to 54mb within 10 days.
DLM reset and the line was at 76.6mb for 5 days, right now it is at 70.6mb.
You cannot use your own kit with Vodafone - they do not give out PPoE details, so you will be forced to use their modem/router.
Only time will tell for me I guess. I am only paying £28/month for everything - but as many people will attest, paying the lowest fee isn't always the best option.
If my connection remains stable at 70mb for the next 18 months, then I'll be a happy Vodafone broadband customer.