The movie makes Ben a sympathetic character: his father is dead, he needs money for Harvard Medical, and he is this social loser who has given up everything for the sake of his studies and joins the Vegas hustlers because the possibility of a relationship with a beautiful girl entices him. It's been a few years since I read the book, but I can't remember any mention of Harvard, and I'm pretty sure Ben's father was still around. Also--what I remember for certain--he had a steady girlfriend in Boston with whom he was discussing marriage. In the book, he leads a seemingly charmed life, and ennui and curiosity, not need for money of social acceptance, motivates him to join the group.
Also, in the movie, we don't see the true extent of his deception and immorality. In the book, we are privy to the constant lies to his parents and his girlfriend. And the group's alter egos consist of more than fake IDs. They were heavily made up, and occasionally assumed different ethnicities. The part of the book I hated the most: Ben starts sleeping around with exotic dancers and even finds one he likes and pursues, all the while maintaining the relationship with his girlfriend at home. He goes from being this good, honest kid to a promiscuous fraud in the course of a few months. The ease with which he was corrupted bothered me for weeks after I finished the book.
The movie calls these escapades "gaining life experience." I'd call it being an idiot.
Sidenote: When Ben goes to his first meeting with the Vegas group, he walks into room 4-145, which would be on the first floor of Building 4 at M.I.T. It happens that I spent much of the summer after I graduated from high school working (i.e. reading The Hitchhiker's Guide to the Galaxy) in that hallway. The filmmakers made a reasonable facsimile, though the actual corridor is much dirtier than the one in the movie.
Current song: "A Little Doubt Goes a Long Way," Reel Big Fish (I think this song has a good message for the young Ben Mezrich and for all of us at some point. Consider the lyrics: "I gotta go, gotta go, before I do something stupid.")