My son went to a Padres tryout last summer between his HS Soph and Jr years (he is a rising Sr now). I thought it was a positive experience. About 40+ guys showed up. A couple more about his age, recent HS grads, old HS grads, current college players, and about 4 recent college grads (these were the ones they were most interested in). It was all about business but the scouts were approachable, by the players, and were very polite. My son thought it was great because after the tryouts the head scout took the time to take him aside and talk to him about his game, where he went to HS, what grade he was in, who his coach was, some other stuff, and offered him some words of encouragement.
Based on this one experience, I would say it is worth the time. I plan, if time permits, on having him attend one this summer also.