Obsidian Conflict is a cooperative multiplayer modification set in the Half-Life 2 universe. It features a large amount of new gameplay features on all-new maps specially designed for the mod with focus on cooperative gameplay, as well as over hundred user-made maps following the same formula. In addition to that, You're able to play through vanilla Half-Life 2 all the way up to Episode 2, as well as experimental support for cooperative Half-Life: Source play.

TheSniperFan says

9/10 - Agree (7) Disagree (1)

So I played Obsidian Conflict...
Well, I expected a coop-mod and that's what I've got, but didn't expect it to be THIS good.
My first impression was bad, because there was no server with a good ping back then. However, as soon as there was one I played it for hours...and hours...and hours.
Gameplay: Great. I got what I expected and even more.
Maps: This mod got them all. The realistic ones, the funny ones, the strange ones and the ones you already know from other games. (My personal favourite is called "oc_manor").

However, this mod is by no means perfect.
The two mayor aspects that let me down were these:
1. The quality of the content is everything but constant. You have high-quality contents, medium-quality contents and low-quality contents. The point is that this applies to literally everything in this mod (maps, weapons, skins, models,...). Sometimes you have different "levels of quality" in one map. This should really be adressed for the Steam-release. I think they should overhaul the maps, what wouldn't be much work for the small, fun-maps but a hell lot of work for the bigger ones.
2. The player-models. I think it's OK to let players choose as who they play, but as long as there is the possibility to choose whatever you want there always be people that will choose strange player-models. All I want to say is this: When you play a realisticly-styled map it just plain sucks to see some animie-fan's ruining the atmosphere. I don't want the dev's to restrict it, but why not making it like this:
The mappers add the models to their maps and the player can choose from one of these. This way you would have fitting models to the map, e.g. play as Star Wars character on a Star Wars map, play as Halo character on a Halo map, play as animie character on an animie map,...

To sum it up:
Just give it a try. Go play and have fun!
But, don't forget to install and mount the other games that are supported in the mods options.