I played this with my homie and I loved it. It's just Half-Life but in multiplayer there are some new mechanics like strafing and new gun animations. The guns also seem to be different I don't know why but they do feel nicer to shoot. The addition of a med kit is also awesome I like how you can heal your teammates. The best part is that it is free you don't even need Half-Life so if you have a broke friend you can still have a fun time. The only downside is that you get stuck and can't move in numerous amount of places and my homie even had to kill ourselves or use the No-clip cheat to get out. I recommend to anybody who loved Half-Life.
I really love blasting my friend into pile of gore when he is standing near an explosive. I have played it along with a single person but, co-op makes half-life a real charm. Single player campaign is already so good. On top of that, you get loads of content through mods and all. If you have friends 🙄, you can surely enjoy this.