I finished this game last night after a week of intensive evenings. I don't have a whole lot to add to the extensive discussion that's already taken place, but I'll just post a few of my thoughts below.
In summary though, I loved it. There are a few things about the first game I preferred, such as location variety, but overall this sequel has improved, iterated and expanded upon virtually every aspect of the original.
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I took about 32 hours on the default moderate difficulty, with no assists. A lot of that time was spent exploring every nook and cranny, collecting every bandage and blade and whatever else I could get my hands on. I also really enjoyed the guitar sections, especially the free form practice option, which I did probably spend too long on, as it was perfectly possible to play real songs, and was implemented really well, giving you the ability to pluck individual strings with careful use of the touch pad.
All in all, this is a masterpiece of a game, and one that'll stick with me for a while. As for the future of the series, I don't know. Nobody wanted this sequel, and I expect nobody will want a third. But they're probably wrong, they just don't know they want it. I can't see Naughty Dog failing, and have every faith they'd deliver another knockout game if they felt like returning to this world.