Hi there,
Our son is in a French immersion program (English only at home) and we chose this school not only for the way the program teaches but also because of the many benefits children obtain by learning 2 (or more) languages at a very young age. Studies have shown that while children may seem to lag in the beginning, their brains develop in a different way and usually outperform non-bilingual learning children.
Immersion school children's brain is often more developed in the logic department as well. Students not only gain skills in reading, writing, listening and speaking in both English and the second language but their academic achievements equal or better those attained by peers in English-only programs. Here is a link to a study: http://www.carla.umn.edu/immersion/acie/vol12/no1/bridgeI.... If you just google immersion school brain development, you will get a host of articles and studies that you can read that may better answer your questions.
We have been very satisfied. I think of it like this - how much do I wish I could speak a 2nd language as if it were my first? It would have been so great to have had this opportunity as a child, so I want to make sure to give it to my child if I am able.
Best of luck,
S.