Okay so there's been a LOT of people complaining that the game is highly unbalanced and that you'll enter a game only to find the same species (usually Predators) winning over and over again. Now I can confirm that the reason this happens is because the best players usually play as the predators.
I've played as all three races (found myself to be most effective with the human) and found that there is no inbalance in the races: they all have weaknesses and strengths which turns the game into one long round of rock-paper-scissors (Human Beats Aliens, Predator Beats Humans, Aliens Beats Predators) but this can easily be overcome with enough skill.
Really it depends more on how you play rather than what you play as. Also the one-hit-kill moves are ridiculously satisfying.
So overall a very balanced, and fun game that's varied enough for you to come back to the same map and game mode over and over again.