City of Ashes picks up not long after City of Bones left off (spoilers about the first book coming up): Clary is still learning about and dealing with her new-found knowledge about the world and her place in it as a Shadowhunter, her mother is still in a coma, her best friend Simon is still in love with her, and she is still fighting her feelings for Jace, which are impossible and wrong now that they know they are siblings. Jace is also fighting his feelings for Clary, and as his true parentage has been revealed, he is now under suspicion for spying for Valentine among other things.
Things I liked: It's fast-paced, very readable, has likable characters (Magnus Bane is top of my list at the moment), a fun magic system (I love the tattoos), a major twist I didn't expect, and I especially appreciated the fact that Cassandra Clare didn't spend a lot of time getting new readers up to speed -- she just jumped right into the story.
Things I liked not so much: Sometimes the plotting made me feel like I was reading the transcript of a D&D game (albeit a very entertaining one), sometimes the prose veered a bit too far into the Romantic Fantasy Description Zone ("Jace was glaring at her, his golden eyes bright as a cat's."), the descriptions got to feel repetitive (rather like the Twilight series), and sometimes I felt that I wouldn't mind if the author made me do a little bit of work, rather than having the characters explain every little thing. But I suspect that those are personal taste issues.
It's brain candy, with less angst and more action than the Meyer books (and, in my opinion, a more entertaining heroine -- she's still kind of brainless at times, but she seemed quite a bit brighter than in the first book and she's not a whiner like Bella). Spoilers/theorizing in the comments.