J.
Although I can imagine all of the wonderful things this will give your child, I'd like to speak as a middle school English teacher on the subject.
Before I had my son, I taught in a district with a wonderful French immersion program in grades K-5. The kids came to me in 7th grade as fluent French and English speakers, but 95% of the time their grammar, spelling, and sentence structure was just awful. Because we were beyond those basic lessons, we did not spend much time reviewing phonics or sentence make up, and often the French immersion kids were lost. They'd mix up French phonic sounds and word placement, and were therefore behind everyone else in all writing areas. I had a tutoring group after school, and the majority of those kids were FI.
So~~~~I think it can be a wonderful thing, as long as their basic English skills are taught heavily as well (and this might have to happen at home).
Good luck!