I really enjoyed the first two books in this series. I was a bit ambivalent about the sudden introduction of oppositional gods introduced at the end of book two, but I was willing to see where the author would go, down that path.
I won't say that book three was a bad book, but it was disappointing compared to the first two. There were far too many just-in-time escapes enabled by unlikely plot devices. The constant shifting of character POVs from one chapter to the next didn't work -- I found it distracting and it made it hard to really get engaged with any of the characters, even Vin and Elend, the main protagonists. The only secondary viewpoints I cared about were TenSoon's, because it was interesting to see what Kandra culture was like. Spook's story, on the other hand, bored me, and I got sick of Sazed's whining very quickly. How many pages did Sanderson have to devote to Sazed's angst about faith?
Getting back to Vin and Elend: their relationship really was a bit lacking all through the series. The only time I found their emotions believable and compelling was in book one, really, when Vin actually acted like a sixteen-year-old girl in love. By book three, she's a hardened killing machine (at the age of twenty) and he's gone from bookish, nerdy scholar to heroic warrior-king, and one gets the sense that Sanderson is embarrassed to write any kind of adult romance. I'm not asking for explicit sex scenes, but Vin and Elend seem to idealize one another in an almost platonic fashion. They talk about how much they love and respect and trust one another, but there's almost nothing about physical attraction or companionship or any kind of softer feelings.
The ending was a literal deux ex machina, and while there was enough set-up to make it (almost) believable, it still had the feel of "And then suddenly God made everything all better."
Lastly, I got really annoyed that after making the Lord Ruler a horrible villain in the first two books, suddenly he's almost a hero here. I guess the horrific, brutal, thousand-year oppression of the Skaa was all Ruin's fault? If the Lord Ruler gets credit for all of his preparations to try to save the world from Ruin, I don't think he gets to escape blame for all of the evil things he did by saying it was "because of Ruin's influence."
So, while I would recommend the entire series, and even the final book, I do feel that Sanderson's execution didn't quite come through in the conclusion to this epic.