Ok, I am definitely more into SP (I do not play MP almost at all, having spent years in Counterstrike 1.1 to 1.6 cured me of that) but you still did not explain what makes it so good, and I am curious, particularly when it can be played solo.
Sorry for taking a while to get back to you.
It's a game that's much greater than the sum of its parts. The game creates stories for you to tell your mates, almost every run. On paper it shouldn't be this good, but has some secret sauce thing going on.
The potential for dynamic, emergent gameplay is second to none, largely in part to the way you interact with randoms (and the AI) at any given moment, especially solo. Uneasy alliances are formed with common goals in mind (or not).
The sound design is incredible.
The AI Is really fun to fight, nothing is scripted.
The sandbox the game provides for you is fantastic and can be messed around in endlessly.
Solo is probably the best experience in the game right now, Followed by Duo's then Trios. Trios usually ends in a fight 90% of the time. The game isn't perfect by any means, the inventory management sucks on a pad, but its not a deal breaker. The lighting indoors leaves a lot to be desired and there is some desync at times. Some of the spawns on some of the maps are terrible. The actual combat is more tactical than your average shooter but not best in class, not terrible mind. If you are looking for tight combat akin to some of the best FPS's this ain't it.
It's a milestone in online gaming in my opinion, hopefully the dev's don't fumble what they have here because right now its something special.
Edit: not a fan of posting youtube videos to re-enforce my views but this guy sums things up pretty well if you have the time to watch and are interested in picking it up.