At my uni, only around 40%-50% of students took a prépa and not all of them passed the concours, obviously!
There was no way I could have afforded either the money or the time (like I said I have 3 kids and am a LP), so I did without.
My uni has a very good tutorat system of its own, so there isn't really any need for extra tuition. Lots of the students who took one first time around didn't bother the second time (you can attempt the concours twice), because it didn't really add much, and some were giving out dodgy information (teaching things from previous years that are no longer in the syllabus, and therefore wasting precious revision time!).
I think it can be useful for students who are quite immature and need a formal structure to work efficiently. Most students who enrol do so to avoid any doubts or "what-ifs" if they fail. But most who did admit it didn't really help.
It's certainly a very lucrative business!