"You don't know what you've done.  You've doomed yourselves."

These are the words of the Lord Ruler upon his defeat in book one of the Mistborn trilogy, by Brandon Sanderson.  Book three, The Hero of Ages, hammers that home.

We learn a lot more about the Lord Ruler's motivations, and why he uttered those words, as the characters in this story face the same challenges he did a thousand years ago.  We learn more about the power in the Well of Ascension, about the Kandra, the Koloss, and the Inquisitors, about Allomancy, Feruchemy, and Hemalurgy.  We even learn about Zane's insanity.

Yes, Book Three answers pretty much all of the questions raised in the series.  With respect to how magic works, I feared it was going into too much detail, like Midiclorians and the Force in Star Wars.  Sometimes less is more.  However, by the end of the book, I realized the author had to include the details he did in order to show how the various schools of magic tied together.  It wasn't just explanation for the sake of explaining.  Understanding the plot kind of depends on this knowledge.

In fact, by the end of book three, I saw just how purposeful the author was.  Many details that are seemingly inconsequential throughout the series end up having meaning.  This trilogy was well planned and executed, and anyone interested in just what Mr. Sanderson was thinking can read his annotations here: http://www.brandonsanderson.com/annotation/  All three of the Mistborn books have chapter by chapter analysis from the author.

I mentioned in my review of Book Two, that I felt it could have been more self contained at the end.  Brandon Sanderson actually addresses that in his annotations:

Quoted Text

As you can tell, I'm using this last section of the book to set up THE HERO OF AGES. I didn't want to do this—I wanted all three books to stand well on their own. However, the events in the third book are just too large to deal with in one novel, so they spilled over into the end of this one. I actually began foreshadowing a lot of these things in book one—they were just easier to hide then.

Only one question remains for me at this point.  In book two, the mist form (or mist spirit as it's called in the annotations) does some pretty contrary things.  I can't say which things due to spoilers, but I'm not talking about the obvious one.  I must be missing something, or misunderstanding something, given how well planned this story was.  I'll be searching the annotations for answers.

I really enjoyed some of the new characters, but I found the brooding nature of the original characters getting stale.  I understand they were all experiencing serious doubts about their actions and beliefs.  One doesn't just shrug that stuff off.  But I felt we spent a little too much time inside the heads of people questioning their worth, and ended up skim reading the majority of those parts.  Only Vin seemed to maintain the ability to question herself without going on and on for pages.  Perhaps that was the point.  She's a much more decisive person.  As a reader though, I tire of reinforcement through repetition, so found the other characters' moping annoying.

Maybe I just wanted to get to the action.  

There certainly is no lack of action in these books.  Any time a Mistborn downs a vial of metals, you know you're in for spectacular battle.  If this series ever gets the movie treatment, it would be a sure-fire hit.

So, do I recommend reading The Hero of Ages?  Absolutely, if you've read the first two.  Events in this book, I feel, can only carry the appropriate gravitas if you've been through the events leading up to it.  That requires reading at least book two beforehand.  And since book one is still my favourite of the three (probably because it is the most self contained), I would thoroughly recommend starting with the first one.

How about the trilogy as a whole?  It's great, although by the end it starts getting into areas we really can't relate, so I felt a bit disconnected from the point-of-view being presented.  I still enjoyed it and think you will too.

For the trilogy:
The Good: Well crafted story/stories.  Believable characters.  Fantastic battles.  Finally a series with a decent Heroine.
The Bad: Copious moping from the characters in book three.
The Ugly: I really can't think of anything that deserves to be in this category!