K.N.
My HS junior was in a Spanish immersion classroom from kindergarten through fifth grade. It was TERRIFIC for her! In gr.6 we moved her to a private school (for reasons totally unrelated to her immersion experience) where she had no second language until Latin in the 8th grade, then Latin and Spanish in HS. Latin was a piece of cake and she totally maintained her fluency in Spanish despite the break.
Being totally conversational in a second language is such a confidence builder. Plus, for our fast-brained girl who is ADD and was reading by 4 yo, it slowed her down a bit and helped her develop some study skills and an ability to pay attention in a classroom early on (sometimes a challenge for this type of kid -- and her teachers).
Can't say enough good things about it. By the way, it totally did not hinder her English reading and writing abilities. She just got her first SAT scores back and scored over 600 in critical reading and math and over 700 in writing.
It takes a bit of faith on the part of the parent (because, at least in our program, there was no English until 3rd grade, except in classes like Art, Music and PhysEd). You also sometimes need to be patient with well-intended relatives who are concerned about the lack of English. Hang in. It was SOOOO worth it for my daughter!
One last thing: she, who can be a little reserved about new experiences, is going to Nicaragua next month with a service group to work in an orphanage for a week. She can't wait to work with native Spanish speaking children and feels (rightly so) that she'll be able to help her friends who can't speak as well communicate with the kids.
One more last thing :) -- if you're worried about learning subjects other than Spanish, that was a non-issue in my daughter's immersion class. As her kindergarten teacher explained it, the second language is the method of instruction, not the object of instruction. They learn the second while listening and speaking -- not through translation. For example, the first time my daughter ever had a math class in English was when she transferred to a different school in the 6th grade. She now is in the honors track for math in high school and is doing well in pre-calculus as a junior. When she "thought" math for homework or whatever, she did it in Spanish. I remember her science class when she was in first grade; they focused a great deal on weather -- but in Spanish. One of the advantages about starting so young is that vocabulary is still building. They work on acquisition, not translation, so the chatter back and forth is in Spanish. Until you see it in action, it sounds very, very foreign -- but to the kids, it just becomes how school is.