I have a PreK child, and the full immersion is no problem. DC was coming from an pre-school program that had some ESL students, so was already familiar with the concept of people speaking other languages.
The classes are small - 17 students & 2 teachers - and the families seem really nice. We've gone to a number of birthday parties, and so had the opportunity to interact with other parents and see the children interact as well. They seem to adore each other - a very close knit little group. I don't know very many parents names, but I hear so much about the other students and I can tell you all of THEIR names. This gives me insight into how comfortable the children are with school, with their class, with their teachers, and how happy and well-adjusted they are. That's important to me, because though I love watching the acquisition of Chinese, I also want to be re-assured that there is age-appropriate social, behavioral, and emotional development taking place as well.
Our student-led conference was today (it was a joy) so I learned more about the daily routine. My child gave us a walk-through of the day, and did various activities (math patterns, daily poems/songs, reading/writing, etc.) and then a short teacher-conference. I learned that at this point in the year, the expectation is now that they speak Chinese at snack and at lunch. My DC is a chatterbox, and his English skills are very advanced. This apparently necessitates frequent reminders to speak Chinese while in the classroom.

The good-humored teasing from the lead teacher that she was going to do something unexpected and speak to him in English - but only for today, so the rules are still in place - was amusing. The discipline and classroom management is clear, and yet kind and gentle. DC gave her several hugs, and was squirming with happiness to watch his worlds (home and school) interact - all about him.
In terms of the activities, it was like visiting any other pre-school with a lot of cooperative play, counting games, daily songs, etc. except that it was in Chinese. It's pretty amazing to watch your 4 y.o. read aloud in Chinese and then draw a picture of what he just read (!), but you remind yourself that memorizing characters and decoding English phonetics are two distinct skills.
I agree with the above poster who said it's actually probably less disruptive than a 50/50 model, because there are fewer transitions - same teachers, same language, same classroom every day. We'll see what next year brings. I love PreK so much that right now I wish it could stay this way, I'm glowing with pleasure at everything YY offers him.