Got to say, the game was one of the best 5 games of 2009, alongside Demon souls, Infamous, Uncharted and Assassins Creed 2. Yep, it was, IMO, that good, better certainly then batman, MW2, and the like all of which are usually as quoted as contenders for 2009 GOTy.
It is for me, the best FPS out this generation of consoles, I have put more time into it then every other (apart from Killzone 2 which I still haven't started thanks to this game!) FPS *combined*; this could be it the guns felt more orginal fun, the stat tree and levelling made it worth me playing these missions, the sheer fun from Co-Op goodness, the hilarious NPCs (easily one of the funniest games in *years*), and the RPG nature of it was light enough so that the shooting was actually fun (note: unlike fallout), whilst having the levelling of character/stats, the stat tree with the special abilities, etc. The only flaw for me was thought he story-telling and characters were great, the narrative arc wasn't as strong as it could have been considerings its RPG elements - but thats not to say it isn't still much better then 90% of your standard FPSes.
Oh, and re: quests online - the way it works (which can be quite anbnnoying) is that certain quests have a pre-requisite for you to complete, whilst others don't (or merely ask you can get to the area where the quest is *given* to you). When you jump into Mp, the shost determines the quest that is active, and as long as you have completed any pre-reqs, you complete it with them (unless of course you already have)(. this does mean that to get round the "do I have this quest in my log", when you log in to the game, your quests are kinda-synced with the hosts, so any quests they have received and not completed (including of course the current active one) automatcally get added to your quest log, whether you like it or not (this is the annoying part as it can bog your list down with stuff you turned down originally); but ensures that when the mission is completed, you get the rewards znd completion.