lies, damn lies and usage limits
I'm pretty pleased with my ISP, newnet, because they were very honest when they reduced monthly traffic allowances having effectively asked customers whether whey wanted to pay more per month or have allowances reduced because it wasn't possible to maintain the expected high quality of service (un-shaped, un-phormed, un-filtered, un-mangled) that the customers expected.
the result was that customers decided on a lower usage limit. service quality has remained high*. we all get usage graphs so we know what we're using, and if we do go over the top there's a reasonable additional charge per GB.
TBH, if I don't torrent too much and don't hit iplayer too often I don't come close to the 20GB limit - in fact, towards the end of the month I usually make an effort to use it up!
A friend was on the talktalk service and performance was very variable, usually poor, he changed ISP to same as me, choosing to actually pay for service, and it's transformed his experience - I can even VNC into his computer to fix problems, which was simply not possible before!
I'm not affiliated with the afformentioned ISP, I only mention them because they seem to be one of the few honest organisations who don't promise top level service at unsustainable prices.
* I get a genuine 8M line speed and I get very close to theoretical max when downloading any time of day or night; I also get consistent low latency, and can't remember any outages.