Just going by the reviews on one of the websites:
Argh, I can't find the one I was looking at now, but I had a site which had top ten and bottom ten ratings. Tiscali appeared to be in the bottom 10 for everything. O2 appeared to be very high.
We've never had any problems with them (apart from when all our emails didn't get forwarded to Googlemail and disappeared for a couple of days) except for them not letting us take advantage of their new deals, but they do seem to have a very bad reputation, and their helplines are in India which can cause comminication communication frustration.