I've heard too many bad things about BT broadband to ever commit to a 1yr contract with them – which appears to be what they want. Hence when we dumped Tiscali a couple of years ago, we chose Zen for BB and stuck with BT (option 3) for calls (which I think is unlimited free calls peak/off peak, 0800/0845, and reduced mobiles, etc).
Personally, I don't like committing 1yr up front to something I don't believe enough in. Zen, on the other, offer a month to month contract, so if you don't like it then you can move on. Two years on and only a couple of hiccups, but they were resolved properly by English speaking people in England who answered the phone within seconds, and called me back when they said they would. Might cost a bit more, but I get peace of mind.