Left Behind was included in the digital version I bought on PSN and while it only has been a short while I finished playing the full game I still wanted more. Naturally I started up Left Behind and was surprised to see that the quality of the content was as high as the original.
This DLC shows us two things. First how she managed to take care of Joel when he was wounded and second we get to meet Riley and how Ellie was bitten. The Joel part was the more intense one since it immediately put you back into the action and then some. The Riley bit served more to give background on Ellie. To be honest while this was nice I was not completely enthralled by it. The gameplay here was not that interesting to me nor was the reveal. Still it is nice to get some background on Ellie and how important Riley was to her. While playing The Last of Us I was wondering what happened to Ellie and Joel after he got wounded. And Left Behind shows us. But, which is a small one so don't get a Hissy Fit. It mostly is filler plot. It does not really reveal new and interesting stuff. However it does put Ellie in some nail biting situations and a chance to get into it once more and I am all for that.
So yes, I do think this is worth playing. Do I think it's worth buying (if by chance you can't get it included)? That really depends. I would not have bought it, had it been over 5 euro's. Since for that you only get more of the same with mostly filler and pointless plot. If you are expecting something new and surprising then Left Behind is better left alone.