I think my issues with them was being unable to talk to them. I rang at a weekend unfortunately, and it cost me about £4 to sit on hold for 30 minutes. I had to ring from my landline as mobile coverage at home is shoddy. I had bought one of their "Home Cell" devices too but it was pretty useless.
There's no way to end a contract online, only in person by phone. I had several fruitless chats with customer support once they were chasing me for money but they were on-script. When I stopped my Direct Debits, I hadn't used their sim card for about 12 months and the handset was fully paid off. I think there's only a couple of operators who stop billing for the handset after it's paid for, one of them is O2. It you do stick with Vodafone, certainly check what you should be paying.
The only positive experience on Vodafone was in Spain, where I bought a cheap PAYG card pre-loaded with heaps of fast data that lasted us 2 weeks.