Apparently, some have fun with the singleplayer even though the game is about 30 min long with no major story at all going on. I don't know much about the ai but I have heard it was considered as sometimes glitchy. As for the graphics and realism...they are decent. The textures are a bit bland and the guns feel very stiff but that's i guess just preference in the end.
Just my word of advice; if you're thinking of purchasing this, spend your money on another game. There's loads of fish in the sea...especially when this game isn't even on steam anymore, at least where I live