WARNING: This review does NOT contain spoilers, but the comments on it do.
So, it seems a lot of people didn't like this book. Some say it's too much like its predecessor, others say it's too different. I suppose it depends on what you focused on... but I loved it.
I can see the "too similar" argument somewhat, as there are some rather similar plot elements at work here (that is, beyond the idea of a heist itself). Personally, though, that didn't bother me. If every book in the series ended up kind of similar that would be a problem, but I'm fine with the second book feeling a bit like "another episode" as the first book was hardly enough of the characters and settings for me. From everything I've heard, the third book will be a different beast entirely, so we shall see.
Back to book two, the world-building, while not giving us any place quite as cool as Camorr, is still top-notch. The plotting may have been a bit weaker, but it still felt as though everything came together quite nicely at the end (except for a little cliffhanger), and the book could stand mostly alone, although reading the first book would certainly be preferable as a continuity, if not necessary, is very definitely there. However, characters and personalities are what I always pay the most attention to, and as another set of around 500 pages with characters I'd already come to love, Red Seas Under Red Skies did not disappoint even slightly.
This book is largely about Locke and Jean's relationship, and as I'm always a sucker for well-drawn friendships (Three Musketeers style and the like) it completely worked for me. We learn more about each character, see more of their flaws, and meet a great gaggle of new ones (PIRATES!).
This was fun, twisting, fast-paced, touching, and gave the slightest impression as to where the series actually intends to go--and it's a direction I'll be happy with.