And here I am, just on cue, ToadMum
I have used them every two months or so since January with DS3 (5 in total) and I think they have been effective to measure progress and weaknesses objectively for us as we are not using any other external agency for tuition etc. HOWEVER, my reasons for using them so regularly have been very much due to DS3's personality and my worries about whether he could handle having to deal with the whole exam environment.
I was more confident with DS2 so only used them twice at £45 each. I think the more you are insecure, the more you feel DC needs it - and the mock providers target this insecurity when they advertise. I have seen some for over £100 too
They are useful but I would not say they are necessary. You can buy CEM style practice papers now for £10 (which you couldn't do when DS1 and DS2 sat the exam) and produce similar results if you are strict with timings. This way, you even get to go through the actual paper too. The biggest drawback with the mock centre I used is that you don't even get to see the paper - although you can pay another £15 for the privilege of knowing which areas they were stronger/weaker in.