Exciting, mysterious and funny. Clare fans won't be disappointed.
My first impression of the book, though a lot of things happened even in the chapter 1, was that it started off somewhat slowly. For me, at least. But then it seemed to get more intriguing and dynamic. My second impression was that there were a great number of parallels between the Cities and this book - for instance, Will and Tessa really talk only in Chapter 2, and in Chapter 3 Tessa awakes in the Institute, just like Clary in City of Bones.
There were a great number of other similarities, too, the most startling of which is how identical Will is to his grand-grand...something son Jace. I swear, it was only a change of name - Will acted and reacted exactly the way Jace would have in his place. That was... strange, mind-boggling even. Tessa was also in many ways similar to Clary.
I found the other characters original and intriguing. Both Charlotte and Jessamyn would develop interestingly with the books, I'm sure, as will Sophie. And though it's obvious that Tessa should be with Will (at least I see it that way) Jem really is a jem! He's sooooo cute. But he's not without wounds, so... hope things turn out for the better for him.
Magnus made a welcome re-appearance, or rather pre-appearance. The villains really unnerved me, which was good, and though I guessed some outcomes, there were still great twists and turns to the plot.
What's most important, for me, is that I finished the book thirsty for more. I must admit, I like the Cities better, but the Infernal Devices seem to have a number of gems in their plot and the first installment leaves the reader with a great number of the most intriguing questions unanswered.
Now all we want is the second and third book, Miss Clare :)