What really frustrates me with them is when you get an operator with a poor standard of English, or an accent so heavy that you are struggling to pick out the relevant words... I have been known to hang up on them and re-dial. Having said that, when you get to the second tier of support, i.e. when they start ringing you back, then they are pretty good.
The CS issue alone is not enough to make me want to look elsewhere for my broadband, as the connection rarely drops out (only occasionally in the small hours when I'm in the land of nod), and the general throughput speed is generally satisfactory.