I've been on one. It was better than getting points - the cost is about the same as fine, or a bit more, but at least you dont get the 3 points.
It served no purpose for me. I knew I was doing 58 in a 50, what I needed was a "rozzer in the bushes" awareness course. And it wasn't long after we'd been rammed by the 3 druggies and I couldn't get a policeman to come help us so frankly I really wasn't impressed, to my mind they should have been out catching criminals and keeping my family safe. I don't condone speeding, especially in residential or built up areas but I got caught on a quiet country road you could have safely done 60 on.
I think they're aimed more at the old people who just sail everywhere at 40mph, be it on a deserted country road or past a school and have no real idea what speed they're doing. My father in law racked up 12 points doing this.
The only way to get through them is to turn up, answer the odd question during it, so they know you're paying attention - they can fail you if they think you're not.
And don't try and debate speed limits - they won't listen. You are there to be talked at.