Fine in principle - but oh dear Lord, the reality!
Interesting to hear about other people's orders being mysteriously "cancelled" without their knowledge - this happened to us as well.
Due to problems with both a defective BT Vision box AND a defective Home Hub, plus fluctuating line speeds, plus having our problem logged as a broadband issue despite repeated advice to the contrary, we endured several months of constant problems, before ending up with no "on demand" service for 23 days and no BT Vision service whatsoever for 16 days.
We estimate that it has taken 45 phone calls, and 19 hours of phone time to resolve the problems - which included 4 visits from engineers which failed for various reasons (#1 and #2 didn't ring the supplied mobile number to gain access to the property, #3 was the wrong kind of engineer, #4 swapped the BT Vision box but left without checking the service).
Full grisly details at http://troubled-diva.com/labels/broadband.html