I used them to bring my DS up to speed in his maths, as he seemed to have fallen behind somehow. I thought they were on the expensive side compared to a private tutor, and its not one to one tuition. I disliked having to pay for the weeks when we were on holiday. But, having said all that, they did help him and when they thought he was at the same level as the rest of his class they were quick to say he didnt need any more as he wouldnt want to be in front of what the class were learning. Overall they did help.
So far, so good. It's early days, 3 weeks before then end of term and he'll go 3 times over the summer before getting back into it in Sept (Y6).
It is 1 hour 20 mins per session but I like the fact it isn't 1:1. The tutor sets DS a task then leaves him to do it whilst attending another DC, then, when he's finished, the tutor checks it, corrects it, sets more and moves on.
It is expensive but I was shocked at how few private tutors there appear to be in this area!