I just finished playing the PS4 remastered version. Wow! From an artistic perspective, I found the game to be superb in all respects: storytelling (aside from the fact that I don't think you can vaccinate for fungi), cinematics, dialogue, voice acting, animation, set pieces, music, sound, you name it. It even was emotionally engrossing.
From a gameplay perspective, I found it engaging and challenging (even on Normal difficulty), though I became impatient with the stealth eventually. On some of the longer parts, I would get frustrated when I would painstakingly use stealth to avoid or kill enemies for twenty minutes only to end up dying and having to start the whole thing over again. I found myself just trying to run through the stealth parts in later levels or, if that didn't work, use firearms to take out the humans.
The ending was powerful. I wasn't sure what to expect. I found myself wanting to have a say in the outcome and am still not sure I'm totally happy with the way the game ended. The ending itself was well done; I just wish there was a way to chose. It was especially tough being forced to kill the surgeon and assistants. I can't imagine there are too many of those around. I guess the point of the ending is that Joel is a flawed character and he can't bear the thought of losing another young girl that he was close to, and he's content just to live out a small life in Jackson City even if it means that no cure is found.
Finishing Left Behind very soon after finishing the main game still left me wanting. The Last of Us has an incredibly powerful finish and the DLC doesn't get anywhere close to that level of gravity. I'd say it would be better played right before or somewhere within the David section rather than after finishing the main game, but the flashback parts of the DLC carry more weight after hearing some comments Ellie makes very late in the main game. Seeing the flashback before hearing Ellie's comments wouldn't really spoil anything, but I think it is more intriguing to hear the comments and then go back and see a bit of what she was referring to afterwards.
Left Behind is good, but it's really impossible for it to match the main The Last of Us game. It's probably better to play it after at least a few days have passed after beating the game so the lack of the sledgehammer blow in the DLC's story doesn't disappoint you.
Story spoilers follow...
I've been thinking about the ending of the game and find it sad. I understand that it would be tough to reconcile saving countless (but faceless) humans against losing someone very close to you again, but man the move Joel made was incredibly selfish. Not only did he prevent a possible cure, but he murdered several (presumably precious) doctors.
I guess it's really a lose-lose situation for Joel. Either he saves Ellie and has to deal with the guilt of possibly spoiling humanity's salvation the rest of his life or he lets Ellie go and has to deal with the terrible feeling of suddenly being alone. It would be even worse if Ellie's brain did not lead to any sort of progress against the fungal infections.
The game is better for the controversial ending, but it really sits heavy in my mind.