I felt that this book did NOT have Second Book Syndrome. I felt that Matched drug on and things were inserted that didn't seem plausible at the time (i.e. Cassia's sudden support of open rebellion and the poetry seemed out of place).
Most second books delve too much into characters heads and they lose track of the numerous goals or plots still going on around those characters. This wasn't a problem in Crossed. I felt that the insight was deeper than most and felt like I was trudging through with them. The outside world's tragedies bound them to their respective missions.
The only problem I had with this book, really, was that it ended quite suddenly and suddenly we're back to, essentially, square one. That being said, starting on the last book as we speak.