Wrapped up Middle-earth: Shadow of Mordor last night, and can happily report I enjoyed it from start to finish, 30-odd hours later (although it could probably be finished in half that - I enjoyed the open-world emergent gameplay, so did more of that than I needed, plus all the side missions and collectibles and whatnot). The nemesis system is the extra element that brings what could otherwise be 'another open world game' in the Assassin's Creed/Batman: Arkham City' template (so a good but not a stand-out game) up that extra level, by working the emergent gameplay into the core of the story progression.
The only thing I could think might be a black mark is that it might be a tad easy for people looking for a challenge, although you can up the challenge by choosing to use less powerful runes on your weapons, or choosing strategies that are harder (ie, not 'thinning out' packs of orcs with your bow before wading into battle, or not branding people other than those required for moving the game forward). Even with this, there were difficult moments all the way through, but like many open-world games, the toughest bits were generally in the first third or so.
Oh, and the voice work by Troy Baker, Nolan North and everyone else is excellent as well. We've come a long way from the cheesy PS1 voice acting

. The script is also very good, and even when they re-use lines from the movie it's in context, it just grates with me

.
Overall, I can comfortably recommend it to fans of the AC or Batman games in terms of mechanics, and fans of Middle-earth in terms of atmosphere

.