Some of the Vodafone backhaul links are clearly overloaded. If you find that happens, it could be prudent to take advantage of the option to cancel the contract in the first month without having to pay an early termination charge.
I'm on one of these links which is overloaded, but not enough for VF to upgrade it. I went though an interesting conversation with one of their customer service guys, and he basically told me the best bet was a leave and use another provider. They would even pay my migration fees. However, I opted for an upgrade to the upto 78service, which seems to work significantly better than the upto 38.
During the day it is speedy and good, and in the evenings you can notice some lag. Single thread speed tests can be pretty nasty, but multi-threaded it seems fine. Not so great for streaming tv....
The CS guy said you should be able to ask your proposed new broadband supplier if they are experiencing backhaul link issues before signing up.... I keep meaning to pop into the Vodafone shop in town and ask them.