First time I was called I was self-employed, my wife was on a career break so I was the sole breadwinner, and we weren't claiming anything other than child benefit - so, as you might imagine, I really did not want to give up salary to do it. I was forced to, and it was a great experience.
Ironically a few months later my wife was called up and she got out of it on the basis that she was bringing up a young family - the one my self-employed income was supporting when I was made to do it!
Then about ten years later I was called again, This time I went more willingly and it was a crap experience. I turned up each day for two weeks and didn't hear a single case.
So my impression of the whole process is that it's a bit crap, inefficient and hit-and-miss; but, if you can get on a good and short case, it can be worthwhile.