I have 3 kids in French schools and I'm honestly delighted with the standard of education they're getting here, especially when I compare with what my friends' kids in the UK are doing - and above all, the prospects for young adults leaving the system! I'm also a full-time student and extremely impressed with the knowledge and self-assurance of the young people I study with! The careers guidance also seems to be excellent here!
Of course there are good schools and bad schools. We have moved several times and our choices of school depended on where we were living. The small village school (in a wealthy commuter village) was definitely the worst! All 3 stagnated there and the younger 2 were quite behind when they arrived at their new school three years on.
The younger 2 are now in a large, inner-city state primary where the teaching is EXCELLENT. Not as many extra-curricular activities (skiing lessons etc), but they do sport 4 times a week and have 3 hours of Italian lessons with a native speaker. Both are bright kids and are being pushed as hard as they need to be.
My oldest is a weekly boarder at a Catholic secondary school (we're Jewish, but the religious content is negligeable, vs the excellent teaching). She was struggling in her state school last year as she's highly gifted, but with weak social skills, but she's thriving now and getting excellent results :)
Re costs: Boarding school (internat) costs ?350/month (includes school fees, residential fees + meals and extra tutition where needed). Travel to/from the school is free (50 miles away - conseil général provides a bus pass).
When I had all 3 in a private primary, it was ?95/month for the whole family (?45 for one and ?75 for 2, I believe). Meals were extra at around ?3.50/meal (if enrolled full-time - occasional was dearer).
In the state primary I currently pay just ?0.73/meal, as I'm a single parent on a low-income and the city council heavily subsidises this kind of thing!