You do have to install their dreadful software, but I'm told that, with a bit of ingenuity, you can ignore most of it afterwards.  Haven't used it myself; I'm with Freeserve "Anytime" currently (£13 a month, no call charges) and experiencing no more problems than I've had with anybody else; as Vindelis says, just avoid the time when the schoolkids are all online.

Malcolm