Depends if your exchange is good and robust, and how far away from it you are....
A lot of the big companies are pretty rubbish when solving connection problems and I experienced serious 'death by Indian call centre' with Orange before leaving them last Month.
I've moved on to Freeola who are an Entanet re-seller. Costs me £35 a month which is not the cheapest but it is solid as a rock and I get free mail forwarding and domain registration.
Worth the money for the reliability IMHO.....
There's a lot of review sites out there, but also speak to your neighbours! If they have a good experience with an ISP then you stand a good chance of getting the same if you're right next door....