I did enjoy this book.
It was well-written and the characters were drawn sympathetically.
The ending was satisfying and resolved all the the time-slip stuff in a way that made you go "uh-ha - of course - now I get it!"
But despite all the things it had going for it (including New York in the 70's and copious mentions of A Wrinkle in Time) it failed to move me beyond the appreciation level.
I can see why this book (and author) is popular with teachers. She ticks all the right boxes. But that extra spark that I look for in a book was missing here - I can't remember the names of the characters even though I only finished it yesterday - it took me several weeks to read as I kept putting it down in favour of other more gripping stories and I didn't learn anything new.
This would be a great book to give as a present to a 10+ girl that you don't know very well - her parents wont be offended by anything about it and the young girl will enjoy it (she just wont love it).