Been with Telewest -> NTL/Telewest -> Virgin Media for donkey's years now, with not a single problem.
Recently, though (in just my area, bah), they've started to throttle download speeds during 'peak times', granted only as a test basis, but still. If you download something at full speed for a given period (very short period), download speeds goes from 1.1MB/sec to ~400kB/sec. Not ALL bad, but when you have an 8gig game to download (CnC3), it gets a little annoying.
When is this 'peak time' you may ask? That would be from 4pm to 12am. In other words, most of the time I spend on the goddamn internet. :(