Warning, contains spoilers.
The series was interesting, but in short, extremely underwhelming. There was so much set up for epic end of the world scenarios, but they didn't go anywhere. With a name like THE DEEPNESS, glass windows depicting a eldritch horror with writing black tentacles, and a diary that goes on at length talking about how armies are useless and it's destroying the world, it turns out to be... a little bit of mist. The Deepness comes back, in that it gets a little misty during the day. There was a lot of setup, and it went nowhere. It turns out that the mist is actually caused by the good deity too.
Normally I don't mind a good amount of introspection, but there's so many times the characters have lengthy internal monologues during action scenes or when they're jumping off walls. It really stops up the action unnecessarily. It got worse towards the last book.
And the most disappointing part was the ending. The trilogy was perfectly set up to have a good ending, but it basically seems like so many important characters made it to the last chapter just to die, while a minor recurring servant character randomly ascends to godhood to save the world. The deaths of Vin and Elend, the former ascending to godhood then dying, and the latter, who burns a stockpile of Atium and nearly fights off an army, then dying, didn't seem to fit. Don't get me wrong, I've read all the ASOIAF series and I'm plenty used to characters dying, it just seemed so random and unnecessary. As far as plot goes, this is the most unfulfilling ending I have ever read.