It's legit, I've won a few matches but nothing major. You can deposit however much money you'd like. Then each match has an entry fee, some are as little as a dollar, some can go up to like 25, 50, 500 depending on if you're a high roller. Each contest is a little different, but usually they have anywhere from 300-3,000 entrants and the top 10% or so get paid out money that goes back into your bank.
If you enter a $1 or $5 contest and manage to finish in the top 10 or 15 you'll usually win like $10-$25. It's fun, a little different because it's all salary cap based, but if you're a fantasy junkie and think you have a line on some good sleeper matchups, it's worth a shot.