Flawed single player. Only buy if you intend on playing with a friend.
This should be an amazing game, but your partners ai completely tanks it. It is beyond useless. She chews through ammo at least twice as fast as a human player. Instead of opting for one hit kill melee move on stunned enemies, she always pops off another few rounds instead. She constantly runs directly into enemy fire which forces you to use precious healing items because if she dies, you get a game over. The one good thing about her is that she has the ability to heal you on the fly, saving you time and possibly a couple of deaths. Her horrific decision making is frustrating, but can be overlooked n normal and amateur difficulty, however, if you plan on playing anything higher than normal difficulty, the computer partner will absolutely ruin your gaming experience. She will die quickly, and often, especially near the end of the game. I don't mind dying in a tough game (in fact a lot of the death scenes in this game are very gruesome and cool to watch), but I don't like cheap deaths and feeling cheated, and that's exactly what this is. 90% of the game overs I got on professional and veteran difficulty had nothing to do with anything I did. You can play online but a lot of the time the servers are ghost towns since this is kind of an older game.
If you intend on playing this with a friend, or don't really care to complete it on harder difficulties then I would highly recommend buying this game. It's lots of fun and a great experience. If you're a completionist playing solo, save yourself the time and frustration and get something else.