I've been with Supanet for 6 years this month, and yeah I can confirm that between 4 and 5 years ago they did have problems, especially with a practice of connecting you to a french range of IP addresses ( you always knew it was french since if you typed ebay into google you got ebay.fr site, and lots of ads that appear on webpages were also in french ) . The french connection meant you were speed capped, and no matter how many times you disconnected the modem and 'redialled' thats all you got. After 2 days you got UK broadband again for the rest of the week. Their forum boards were full of complaints and advice on how to check your connection, so they just removed the forums altogether.
You might ask why I never moved, but this was before the time when MAC codes were readily available, and at the time they were considerably cheaper than any other comparable ISP for unlimited 2Mb.
3 years ago I upgraded to ADSL Max and negotiated a reduction in cost, and again last year. I rang them up after the 12 month contract ended, said they were not competitive anymore for similar product, and they chopped another couple of quid off. But they have been fine for past 3 years - no problems at all.
try http://www.thinkbroadband.com/ and the forums ( http://bbs.adslguide.org.uk/ ) for more ISP reviews
Iain