I'm with BT and so far they have been good.
I always think its better to go to the horses mouth with ISP stuff. Most other ISP's have to interact with BT/Openreach, so its better to go with them, IMO.
They have as good a support as any. These days no one company really stands out.
Demon => Freeserve => Pipex => Be => Sky => BT Infinity 2