***This review reflects the game as it was when the zombie co-op mode was first released. I will update the review once I have spent some time in the current version. Also, read my note at the end about the Steam DLC and ingame purchases.***
A Call-of-Duty-ish, loadout-based, first-person-shooter with a massive range of weapon and equipment customization. Players are equipped with a "HRV" hud that runs on a meter and is activated to see through walls to track allies and enemies. You can earn a sort of in-match currency or resource that can be used to purchase heavy weapons, such as grenade launchers, flamethrowers, RPGs, and even nano-swords (think Metal Gear's High-Frequency Blade). This resource can even be used to call in mechanized exosuits called "hardsuits" that dominate the battlefield with a high volume of firepower and their heavy armor gives added protection.
The game, for the most part, is not pay-to-win. Most weapons are equally balanced and are all viable. The standard assault rifle that everyone gets is one of the better weapons in the game. The only pay-to-win is that you can use real money to unlock weapons, gear, and gun parts faster. Also, there's the Heavy Assault Rifle, which is OP, but even with real money I don't think you can buy it permanently. Real money is mostly used for cosmetics or bonuses, such as advanced XP / in-game money rates.
The game has plenty of gamemodes to choose from and it feels fair to play. It's also very visually pleasing, as well as being easy on your system. As long as you have a decent computer, you should be able to run it just fine.
On the Zombie Co-op: It feels fun and is pretty challenging. Unlike other games that have PvP Zombies versus Survivors, this is a PvE survival mode where the players face waves of unique NPC's and bots as a team. I don't remember much about it other than you will run out of ammo a lot unless you keep picking up the weapons the zombie soldiers drop, or you have one of the more powerful melee weapons / nano sword things.
Overall, I'd give it a 8.5 / 10. It's fairly solid, but sometimes it just feels a little... less than optimal? Hard to explain, but sometimes you feel like it could do more than what it currently is doing, if that makes any sense. Maybe that's because it's a free-to-play game.
***One thing to note: I would not purchase any of the Steam DLC packs or purchase anything in-game really, at least for a while. There have been reports of things not working and people losing their money in general. And honestly, you don't really need them, the game's fine as it is without you paying anything.***