I got into The Last of Us pretty late, and only really sunk my teeth into it recently when I was stuck indoors for a second time during the ice storm in Atlanta. After getting a little ways into it, though, I was hooked, and couldn't stop playing. I think it's a phenomenal achievement for video games from many angles: fun, unpredictable gameplay; wonderful visual and aural design; and most impressive, a moving and engaging story. I'm usually not one to care about the story in a game--all my friends will tell you I'm the guy that just keeps pressing A and goes to get a snack during the expository bits--but The Last of Us had me more fully invested in the story and characters than anything since Red Dead Redemption. While something like Bioshock: Infinite had a fascinating plot, I didn't care that much about Booker or Elizabeth in particular, but I cared immensely about Joel and Ellie, to the point that I caught myself gasping when one or the other got hurt.
So that's the background, but what about the DLC? Spoilers be ahead!
It's an interesting way to do it. Left Behind takes place during two parts of the LoU story: during the events of the main game, when Joel is injured and Ellie has to find a way to keep him from bleeding to death, and also before Ellie meets Joel, when she is with her friend Riley that is mentioned in the last few minutes of the main game. The two stories have an interesting interweaving quality. They both take place in malls, one as Ellie hunts for medical supplies while avoiding enemies, the other as Ellie and Riley explore an abandoned mall and wonder what their futures hold. The cutting between these two disparate times is less jarring than you might think, and narratively is crafted quite well. There are some nice almost comic-book-like transitions where a thought or phrase from the present carries over to the past or vice versa. On a bit of a side-note, Gustavo Santaolalla returns to score the DLC and with some haunting piano parts makes the game feel separate but part of a whole in a very satisfying way.
Centrally Left Behind gives the player more background about Ellie. Most of the part of the game spent with Riley is expository, providing some explanation for Ellie's cleverness but also her sense of humor in her relationship with Riley. Much of that part of the game is spent reading jokes, playing imaginary games, and exploring. We see that although Ellie has essentially had no childhood, her friendship with Riley and their last day together sort of fills that gap.
The part that takes place during Joel's brush with death is more action oriented, as Ellie must navigate the mall looking for supplies while repelling both human and infected adversaries. There's little movement in the story here, but it does provide an interesting evolution in the action from the main game: while before the game goes back and forth between sections of infected and human enemies, Left Behind has you fight both simultaneously. This makes for some very interesting battles that make even more use of the simple but cerebral fighting style that LoU was excellent at, as you pit the two against each other by throwing bottles to draw the infected's attention.
Here's where we run into some trouble, however. While both parts of the game are interesting additions, neither are long enough to really justify the $14.99 price tag. Well, I should say that the part in the present is not long enough--the stuff with Riley is plenty long, and at times even drawn out. While the back-and-forth works very well from a narrative standpoint, it makes for some uneven gameplay. This might work alright, but the balance is off--there isn't nearly as much fighting as there is exploring, which satisfies one side of my craving for more, but did not live up to the intense action of the main game. The idea of fighting both types of enemies at once is a smart and exciting one, but unfortunately that only happens two or three times in the short game, not nearly enough to really force you to think about ways to take advantage of it. Having just beat the original game, I found Left Behind to be a little boring at times, though admittedly that could be because I exhausted every possible story bit to be found in the Riley story, reading through all the jokes and trying on all the halloween masks as the girls goof around. It's entertaining and endearing to a degree, but when not balanced with high-stakes action it just felt like this story might have been better served by a short movie or a comic rather than a full on DLC.
From a craft standpoint, though, it is scripted well, is full of great bits of detail, and is acted absolutely perfectly by Ashley Johnson (Ellie) and Yaani King (Riley). As with the original game, the characters feel very real, and that's a product of the clever, casual scripting that lets the characters and the setting breathe as well as the voices of the two lead characters. The story is definitely the strongest part of Left Behind, but unfortunately there's just not enough of it to make it quite the monumentous edition that many fans were hoping for.
Overall, I'll say this: if you were a big fan of the original game and are dying to know more about Ellie's past or how she and Joel survived after his nearly fatal injury...you might want to wait until the price drops. At $15, it is a third the price of the whole game (at Amazon's prices), and that's just a bit much. While I spent probably 20-25 hours on the main game (and am already loving replaying it), Left Behind only took me probably 5 hours max to beat, and there's little incentive to play through it again. It's a decent DLC and will be a great addition when the inevitable Game of the Year (or whatever) edition comes out for half the price, but I wouldn't grab it up just yet.
For those reasons, I would give The Last of Us: Left Behind a B-.