This is from the discussion area of the wikipedia page on opinion polls. It is an analysis of biases between the polling companies by trewa. I know how much our own commentators are interested in the biases of different polls, so this will give some kind of basis for comparison.
Personally I think that land-line based polling as being pretty worthless these days. In my opinion, having a listed (in the white pages) land-line is a characteristic of being older, more technophobic, and being of higher incomes – characteristics of a more conservative voter.