The storyline is nice and fast-paced, and has a few main strands. As might be expected, there's a focus on Elend and Vin, but also upon Spook, who gets a bit more of an active role this time.
Ash is falling thick and fast, volcanoes are exploding and the world looks doomed. By tracking down information the Lord Ruler left behind Elend and Vin hope to defeat Ruin.
Throughout the series I've liked the kandra, and the scenes with TenSoon are amongst my favourites. More backstory about what the Lord Ruler did when he was all-powerful and his relationship with Ruin are revealed, and there are a few especially cunning twists that I never saw coming but which made perfect sense (as well as one or two which are a bit easier to anticipate).
Yomen is a new character who is quite intriguing. He's set himself up as King of Fadrex, but appears to be a pretty decent sort. However, he comes into conflict with Elend, who wants the city (for good reasons) and Mr. Sanderson does an excellent job of portraying how, in crisis or war, it's entirely possible for two opposing sides to be led by reasonable men.
Unlike the first two books, which were almost entirely focused on the capital, Luthadel, most of The Hero of Ages takes place in numerous other locations. This helps the author to flesh out the dying world of what was The Final Empire, which he does to good effect.
I liked The Hero of Ages a lot, and if you enjoyed The Well of Ascension I think you'll like this too. I'm sorely tempted to buy some more Mistborn books (and will be getting the second part of The Stormlight Archive when it comes out), but I'm going to focus on Vanished Kingdoms for now.