I've been with Virgin, formerly NTL for the since the dial up days in 2000ish.
99.9% of the time you will get the advertised speeds with no problems, that said Virgin have been down or on a go slow here more this year than the last 15 years combined.
With an idle internet connection this is the best ive got for the last 4-5 days now:
That happened for a few weeks in November last year also, however eventually sorted it's self out.