Kantor on 31 March 2012
On the one hand, you won't understand the significance of all of your various meetings and some of the game's brilliant events. You won't know why you're so close to Garrus and Liara, or really understand what's going on with the Quarians and Geth, or any number of other things.
On the other hand, you won't be nearly as disappointed by the ending as the rest of us were, and the gameplay is still great (and though the multiplayer is often overlooked, it's a brilliant part of the game).
So, if possible, play the first two (or at least ME2), but don't miss out on ME3. You'll be able to complain about the ending like the rest of us, and who doesn't love to complain?
(Former) Lead Moderator and (Eternal) VGC Detective 
