We sent our son at LFZ for almost two years, if you're aiming to stay here for a long time I wouldn't recommend it for the following reasons:
1) it's an international school, meaning it doesn't follow the regular Swiss calender and schedule. Translated to regular life, it means that your child's vacation will not always fall at the same time as local kids' vacation (in fact, the French school has 3 more weeks of vacation per year), hence vacation activities and day-care may not be available to you. The school days are also longer, making it difficult to participate in extra-curricular activities not organised by LFZ. In both case, it's a pain and makes it difficult to socialise outside the school and integrate in the local community. I know they are planning to organise day-care on Wednesdays afternoons and during vacation, but as of today our friends have to ship their kids to France if they can't take time off and pay for expensive day-care every Wednesday.
2) so far the German immersion program is not convincing. It's rather new, and they will get there eventually, but if you're child doesn't speak German at home she's unlikely to get fluent as it is today. After 2 years, not only did our son not speak German, he had decided it was a terrible language he bever wanted to learn.
3) you will spend your time driving your kid around to playdates, as the families are spread out in the entire region.
4) it looks cheaper than most private schools, but once you add up garderie (not Wednesdays), lunch, bus transport and extra-curriculars, it's not that cheap anymore.
5) lots of kids come and go, if your child depends on stable friendships it might be difficult for her. Our son has a friend who really struggles each time someone he likes leaves the school.
I will keep out of the discussion about the value of the French system and class dynamic, because it's a personnal opinion and I know many people who are very happy about the school. But we hadn't thought at all about the practical implications when we picked that school.
If you want to invest in a softer start for a reason or another, I can recommend d'Insle and Sprungbrett, 2 affiliated private Montessori schools that offer French and have a sizeable population of French speaking kids. After 4 months at Sprungbrett, our son was babling about in German. They are Swiss run, the teaching is bilingual German and English and they follow the local school calendar. If you both work, they also offer real full time daycare between 07.30-17.30, including during the vacations (closed 5 weeks/year).
I also heard Tandem offers French classes, but I don't know the school at all.
PM me if you have any questions.