I LOVE Miller Method. It's made all the difference in the world to my son. And I also have high regard for Dr. and Mrs. Miller. I have not worked with any of their staff. My son is in a Miller Method school. Dr. Miller does video conferencing with the school I think 6 hours a week. There are now 62 kids in the school.
If your son has Pervasive Developmental Disorder (PDD) I would do the evaluation at his clinic in a minute IF I had the funding. Likewise the summer program at his clinic.
In terms of downsides the only thing I can say is that Dr. Miller does functional and educational issues. He does NOT do psychiatric/medication or holistic health approaches. He doesn't discourage any of these to my knowledge (there are many kids in the school on medications, gluten and casein free diets, etc). But I don't believe that he would refer you to any. So understand what you're looking for vs what you're going to get.
And what you WILL get is help with general understanding of how your child thinks, why he/ she is not functioning "normally" and specifically what you can do about that. eg he's excellent with perseveration, communication issues, withdrawn Pervasive Developmental Disorder (PDD) kids, Pervasive Developmental Disorder (PDD) kids with rages, Pervasive Developmental Disorder (PDD) kids who are just blocked on one issue (mine was great with letters but couldn't get that they made words which related to the verbal terms he heard), ....basically all the things we see with our Pervasive Developmental Disorder (PDD) kids.
One of the things that opened my eyes was, coming from a ABA approach previously, that if I approached my child on his level, got into his obsession with an object or activity and expanded it, I could get my child interested in the world around him. So Miller Method turned around my whole way of thinking from forcing my child to do what I wanted with carrots to letting him do what he wanted with minor modifications that I wanted. So my child now spends hours on the computer because that's what he likes to do, but it's on educational games (last night on second grade math). Certainly not exclusively- there's plenty of physical and social activity in his day. He has a well balanced educational program and life in general. But I no longer see an obsession as a negative. I turn it around to work for me. I know this isn't rocket science. If our kid won't eat veggies we hide them in a casserole. But Dr. Miller helped me see that there were ways this applied to Pervasive Developmental Disorder (PDD). And I see Dr. Miller regularly via video conference at school and we work on a new problem each time. eg my son had screaming fits about losing any game to a peer for while which we've worked on and overcome. We think in my son's mind his whole being, his worth, was tied up in winning. so if he lost the game he somehow lost himself. This is the kind of "complex" problem that Dr. Miller has been invaluable to helping with.
The "square" is also great if your child likes heights. The child who's always walking on the furniture at home, climbing to the highest point in the playground,... is the kid for the square. and once you learn to use the concept it becomes helpful at home even without building your own square! and it really does cure toe walking!
Another thing I have to point out... which doesn't apply to you (which I know since we've both been on the board awhile)....is that, in my humble opinion, the prognosis for the child and the value of Miller Method is in teaching the parents and their use of it at home. I started with this Pervasive Developmental Disorder (PDD) diagnosis 6.5 yrs ago. We've been in a lot of classes since then with a lot of peers. While I think Miller Method is the best for MOST Pervasive Developmental Disorder (PDD) kids, I have been dissapointed by the fact that some of my son's classmates don't seem to make as much progress as my son. I've tried to figure out why. And the best correlation I can find to outcome is to the parent's understanding of Pervasive Developmental Disorder (PDD) and the parent's working with the child outside of school. eg when my son had fits at losing we worked out a strategy (myself and teachers with Dr. Miller's input) and I "practiced" at home while they did in school. Sadly, I see so many parents who just send the child to school and think the school is going to make the child OK. Or perhaps they've given up. But those are the kids I don't see progressing.
FYI- Dr. and Mrs. Miller are coming to my son's school to do the 4 day teacher training class in mid-July. They apparently have a couple of extra slots which they're opening to parents. If anyone is interested please let me know. I think the cost is $500. I did this and I can't recommend it highly enough.