First of all - Matthew McConaughey is the lead actor here, and as always, he's pretty damn good. That just needs to be said. Now, the movie itself is based on the events of the Bre-X Minerals gold scandal. People were looking for gold, falsified coring examples, and rode for a while on what was one of the most expensive frauds ever.
So, how true? Well, the usual site article shows a lot of hits and I whole lot more misses. Starting with the fact that the characters in the movie are not even real. McConaughey character is based on specific person, Ramírez' character is based on two people. So, going from there, it already allows the studio to greatly romanticize and embellish the story. And I am pretty sure, that's what they did. I mean, even FBI questioning is made like that for movie purposes only. The positive ending is complete fiction. All in all, one of the most truthful scenes is the helicopter scene, which is as uncertain in real life, as it was presented in the movie. Then again, in real life more people get thrown off helicopters than you might think.
All in all, the movie is powerful and well-made, but can't really be seen as good representation of real events.