Multiplayer is pretty basic and if you only play online, you might be better off with another game.
Endings.. well.. from what I understand there's several things taken into account; things such as reputation (per faction, there's 2 major ones and several minor ones.) Duty (disciplined army-like faction who feel they have to "protect" the Zone), and Freedom (sort of like rebels; the Zone is for anyone to scavenge and you can keep whatever you find). The other factions don't really come into play much. Also the amount of money you have influences your ending. I won't spoil anything, but having a lot of money in this game is not a good thing.
Pro's and cons.. okay. Pro's; A very large playing area. At the center is the Chernobyl Reactor and I believe it's an area about 25 km in diameter, although I'm not sure. A pretty decent story line for a shooter, very realistic shooting mechanics (bullet trajectory isn't a straight line), very nice atmosphere. Very gritty, rusty, run-down environments really capture the feeling of what has happened. Cool guns, several "unique" ones (standard guns with a build-in upgrade, such as increased accuracy or rate of fire).
The cons; Really any quest that isn't directly tied into the main story line is very shallow and rarely consists of more than "Kill [insert name]" or "Fetch [item name]. Although some quests can be repeated (Help fend off a bandit raid), I rarely found myself being bothered by the fact that my "friends" were getting slaughtered out there. The thing is, every time I entered the specific area (in this case, Garbage), I would get the same quest. I did it the first two times but seeing as how most quests reward you with little more than money, it's not really worth all the risk and ammunition. Needless to say, I didn't have a lot of friends at the end of the game.
I hope I've helped.