Many of my family members and friends have visited Medjugorje. I personally do not wish to visit, since I am convinced that the “seers” are snake oil salesmen. However, my father received many blessing there. Like you, I know of several people who have visited Medjugorje and come back transformed. Personally, I have no desire to visit.
I've been to Medjugorje. It has become quite commercialized since starting out as a tiny, remote village in the hills of Bosnia (not far from Mostar). I stayed at the house of one of the seers, who ran a kind of bed and breakfast for pilgrims. She's very nice and I believe she is sincere. There are some others, whom I think are not so sincere and are just cashing in. I was present when one of the seers received the monthly message that is published regularly. I don't think it was fake, but I'm not sure what it was.
Other than that, I didn't experience anything miraculous. I went to Mass and went to confession. Climbed both mountains. Sampled the wine, the beer and the pizza, and the home cooked food at the B&B. It's was all good. Lovely place and no mosquitos.