My son was in a dual immersion program for two years, and there were many things to love about it.
I loved that my son did pick up a lot of Spanish very quickly, and had peers he could interact with in the language. His sister, who also studies Spanish but not in an immersion program, learned a lot of vocabulary from him. He learned a lot of grammar from her. That can be a problem - in some dual immersion programs, kids are left to try and grasp what the rules are in the language. It's true that they do learn by using it, but they can get really confused by things like different verb conjugation patterns if they are not overtly taught that there are patterns and that looking for them will help them to understand. Anyway, I knew that interesting things were happening learning-wise when my kids got into their first Spanish language argument. :-)
One possible drawback to a dual immersion programs is that if important concepts in other subject areas are introduced only in the child's second language instead of in both languages, kids end up missing information that they need. Another possible drawback is that if you have a child who processes patterns well, and the language arts lessons aren't well-designed, a child can fake his/her way through without actually understanding things. (My son was quite good at this. A good teacher will catch on quickly to what the child is doing.)
If the program is not well-organized and well-run, it is possible for other subjects to be neglected because of the time spent on working on the two languages. My son's first teacher in the program we were in did an excellent job of integrating science, social studies, art, and music into the school day in both languages. My son's second teacher did not.
Helping with homework can be a challenge if you do not speak the target language. Google Translate will become your best friend.
Another responder has already mentioned that in a dual immersion program interaction between parents who are speaker of the different languages can be a challenge. At our program meetings, we had an interpreter from the school district office come to assist. Also, we were blessed with several bilingual English/Spanish parents in the group who helped with communication. The language barrier was easy enough to overcome by those of us who wanted to. There were a group of us, both Spanish and English speaking, who made a point of socializing. Even though my son is no longer in the program, we still see these people, and I value their friendship, regardless of which is their native language.
Go to meetings, talk to the teachers, talk to the principal, and talk to other parents. If the program is well-organized, well-supported by the school, and staffed with good teachers, it would be very much worth participating.