I didn't enjoy City of Ashes quite as much as City of Bones, but I was still sucked in by the story. While I'm not a huge fan of Clary, I like many of the other characters so it was nice to see them again.
Clary/Jace just didn't do it for me this time. Maybe the whole Thing just kind of threw a wet towel on it for me, though, I might have found it stale anyhow, it's hard to tell. You know, I get the turmoil in their situation, but the whole flip-flopping back and forth between romantic and non-romantic feelings thing really grated on my nerves. Plus, poor Simon is left out in the cold with the short end of the stick. Clary's all "yeah, sure, we can date and kiss and stuff, but I don't really like you that way." I was glad to see Simon coming to his senses a bit at the end. He doesn't just give up on Clary, but he also realizes he can't be mooning after her the rest of his life while she's off making out with some other guy.
Speaking of relationship:
1) I find Simon and Jace's relationship very interesting.
2) Alec and Magnus are freaking adorable.
As I mentioned, I still really like the story, but I really started seeing the flaws while reading City of Ashes. I couldn't possibly count the times I got pulled out of the story by Clare's writing. It was really clunky and she kept using a handful of phrases or words over and over and over again. Basically, this is one of those books/series I enjoy so much that I overlook the writing and various flaws.
The Nutshell: It's really bad, but it's also really good. (And no, I don't mean "it's so bad it's good.")