We have been with Tiscali for a year and a half now, and I have to say it has been pretty reliable. We get the 2Mb we pay for except for evenings when they do throttle traffic as others have mentioned. We've also had very little Spam. HOWEVER the couple of problems we did experience when they "Upgraded" our local exchange (LLU) confirmed how utterly useless their "tech support" is. It makes no difference whether you 'phone or Email. We certainly won't be taking up the offer of a phone service as well.
My main concern with changing to one of the smaller ISP's - who get such good reports - is how long many of them will be able to keep going in the current market conditions. They are all (as far as I'm aware) reliant on BT's wholesale service, since installing their own networks and LLU equipment is very expensive. If even major operators such as Tiscali are at risk of buy-outs what chance have the small guys?