I'm with pipex and know a few others who have been with them for as long as me (a few years). They usually come out pretty well in 'best of' lists, but a lot depends on where you live. You can have a good ISP in terms of customer service, but live in a densely populated area with loads of demand on the line and your connection can suck. On the other hand you can have a really good connection but have to pay a fortune for it and get poor customer service.
The main thing is probably to find a reasonably good one for reliability of connection and stick with it, then get yourself in a position to negotiate a better deal under the threat of leaving them after a 12 month contract
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I think my current deal is ok (£17.99 a month for 8Mb connection and unlimited downloads). In a few months time when I come to renew it i will be checking the going rate and trying to get them to match the best (as long as my connection remains as stable as it is).
Don't expect 8Mb downloads tho' even if that's what you pay for...on average I think an 8Mb connection may get you 4-6Mb - lower in a highly populated areas, higher in rural areas and close to the exchange).
Many obviously rely on BT for the line itself and you can find yourself in the middle of a fight between an ISP and BT as to whose fault something is. In my experience, with a reasonably well known ISP, if there's a connection reliability issue it's often down to BT...they ain't that quick to sort your problems
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