Just finished Clockwork Prince; surprisingly excellent. This is definitely a book two that does not suffer from mid-trilogy problems. The protagonist, Tessa, is oblivious for romance-plot reasons, and rather characterless by comparison with Will and Jem, but those two are quite vivid and likeable, and the Lightwood brothers become interestingly sympathetic. Loving the Swinburne quotes!
So far, The Infernal Devices is a far better series than The Mortal Instruments, which I gave up on after book three.