Personally, i think they are more of a scam then anything else, but give it a go if you like.
Try doing extra work in some movies if you want to know what its all about. Once you have an agent you can start audtioning for smaller roles and you get too know the casting agents.
If you can somehow swing, try to get into actra, the actors union, every movie shoot has to have X number of actra memebers, even being extras etc..
I did some extra work years ago, my best friend always got the gravy gigs (when they dont have enough actra people on set, they upgrade regular extra's and your pay goes for $10/hour to something like $20. So i tagged along with him, great times and man, its one of the best spots to pick up girls. I wouldnt do it anymore but when your in high school or university, its awesome to meet girls.
the industry is much more about who you know then what you know.