The YAMAKASI are a bunch of kids who practise what is variously known as Parkour and Free Running, a sort of sport/art/philosophy involving the development of skills for the traversal of urban environments in interesting ways (http://www.parkour.com/).
YAMAKASI the film is a Luc Besson production that basically provides a vehicle for 7 of these kids to show their stuff, in the pretence of helping a young kid who needs a heart transplant. It actually feels rather like a kids film, with larger than life characters (a bit of a keystone cops thing going on) and a message about being a bit rebellious but in a good way, or something.
The group are real life practitioners of Parkour, not professional actors - which kind of shows, though not in a particularly bad way - they're not wooden, but don't exactly express deep or complicated emotions.
Given that the film is basically a vehicle for Parkour, it's somewhat disappointing that not all that much of it is shown. There's some building scaling which is impressive but not particularly cinematic, then a bunch of antics where very little "free running" is shown - there are just a couple of scenes which show the potential the film could have had, towards the end. That potential has recently been realised much more dramatically in the film BANLIEUE 13, where one of the founders of Parkour is teamed up with martial artist Cyril Raffaelli for some truly original and sometimes incredible action sequences which show how Parkour could really be the foundation of a whole new action style. It's a shame that YAMAKASI, for whatever reason, didn't seem to know what to do with it. I'm hoping that the semi-sequel LES FILS DU VENT will put the group's talents to better use.