I haven't seen Mesrine yet. I did see Un Prophete. Cassel is a good actor who married way above his station..
You should watch it, it's a great film. It's in two parts, each two hours long. I think longer movies are better, just because it gives you a bigger connection to the characters.
And when the cops, when they assigned a whole army to stop Jimmy, what'd he do? He made 'em partners.