I'm glad the series came to a conclusion that felt satisfactory. Okay, I know that sounds really obnoxious, but it's the only thing I could think to say. Here's the breakdown:
The Good: First of all I just wanted to say how amazing the poetry was. I didn't notice it in the first two but in books three and four it was phenomenal. I also loved the mythology that was built around death. It was interesting and cool. The new characters were built into the mythology. I really liked that. I like that the series has such a definite ending.
The Bad: The big enemy wasn't introduced until book three. That's just confusing. The bad guys that you've spent the last three books hating are the side shows. Not that their death's weren't cool or satisfying, it's just that a totally new element was introduced and it's the series climax. On top of that it wasn't the Book of Moons or the Caster elements we've followed that are integral to the end of the story. That just bugged me.
All in all I'd probably read another series by these guys, which is not what I thought I would be saying when I started it. They're still not on my top list, but you never know.