I have been on BB with NTL since 1999, on the odd occasion I have had no access and slow browsing at the odd time, NTL strive to do their best by their customers and on the whole 99+% of them are happy.
As said elsewhere if your not happy there are many ISP's out there willing to take your money its not as if you have to stay with NTL.
If you have slow browsing try using a proxy's (or changing the one you use) and as for slow downloads, well they do say "UP TO" which gives them a wide scope.
Over all I'm more than happy with the service NTL provides.