It really varies by school. We went to one after one of the coaches DM'd my son and asked him to come. It was local. Complete waste of time. Run by players. They called my son's coach the next day to ask if he might be interested in their school and they didn't even know he had attended. Flew to another one that limits enrollment and has 4-5 one day camps. It is a top 10 school and we thought it was worth it to be seen in a small group. It was great for my son, not sure if it was great for everyone (which the other school may have been great for a kid or two?). There had been no communication prior and the pitching coach immediately started working with my son. He threw the first two innings and then came and got me to tour the facilities and sit down to talk. They started weekly calls at that point.
I would say overall, we went to 3 one day camps. Two were good and a value, if you are trying to be seen, and one was terrible. I guess the value at that school was that my son knew he wouldn't want to play there based on how disorganized everything was.