I think it's actually 870 pages.
I'm in the middle of it, and I will also give no spoilers.
The characters are definitely more developed. Especially Harry, who has hit adolescence full on. It's funny, so far I've noticed that she's used a few words that will send the under-20 crowd to the dictionary (hopefully).
Otherwise, plot structure, themes, etc seem to be what you get/would expect in I-IV. As Supergirl mentions -- it's long. Really long.