I had the miracle blanket for ds, and used it on him fully until he was about 5 months old. Then I started leaving one arm out for him to suck his thumb. We did that for a long time, probably another 2 months. By that point, he was able to break out of the miracle blanket simply due to his size. So, I started leaving both arms out, and still wrapping it around his middle and tucking his feet in. We did that for a while, and then got rid of it completely. But we did wean in stages, so that he was comfortable. They get used to that blanket, and realize that it means it's time to sleep!
I thought the miracle blanket was the best ever! I have two, and I would switch them out, and if he wet through one or spit up on one, then I had the other one to use during naps while the other was being washed.
Anyway, well worth the money if you want to try it.