Generally, you get pretty much what you pay for. I chose Pipex on price and spec, and have 512k with no download etc limits. I have no idea how much I up- and download a month.
The phone line is BT.
Reliability has been excellent: it went wrong in late 2003; the line dropped briefly some time during the night in August or so 2004; apart from that it "just works".
Pipex's Customer Service, last time I tried (2003) was pretty ropey. I've not needed it since. On balance, I reckon I made the right call.