My school runs study camps every year for HSC students in the October holidays just before the HSC starts.
I went last year. I found it to be great. Although I nearly died of boredom [35 hours of study in 5 days will do that to you], when I actually did work it was very productive and without that camp I know I wouldn't have done as well as I did in the exams.
We had all of our teachers there to help us though.
I don't know how useful a study camp without teachers would be [like the Crusader camps]. It's good for getting you motivated to sit quietly and open your books, but after about four hours of studying in one day in that setting it's very easy to lose focus. If you get stuck and no one can help you then it might just lead to you wasting the rest of that session.
It depends on who's running the camp as to what to expect, but at ours: we all sat in a hall, with our own desk, and you worked on whatever you wanted to silently. There were sessions of 1 to 2.5 hours, and short breaks in between or meal breaks.