To be honest, when I bought this book I was afraid that it might either replace my other Song of Blades and Heroes (SBH) books, or be redundant. Still, I bought it, attracted by the game setting (which, from other reviews I'd read over the 'net, sounded quite unique) and the new magic system, which I hoped could be adapted back into SBH (by the way, it surely can.)
That fear proved to be unfounded -- even though Shadowsea is also a fantasy skirmish game (like SBH), the authors have done a good job of making it into a distinct game. The core rules feel like a revised edition of SBH with some stuff added from the supplements, but on top of that there is a lot of new material -- special abilities, rules for non-combat actions, interactive scenic items, magic, technology, and so on. The book includes six pages on background and over thirty pages of force rosters and creature stats, but the whole text is permeated by the flavor of the game setting. I think that maybe it's this blending of setting and specific rules that gives it the distinct feel.
Regarding the presentation, the text is well-written and includes some rulings and clarifications that appeared in FAQs or in the discussion group for SBH. The book also contains many beautiful illustrations that help set the theme and mood of the game. The layout is clean and functional. The detailed table of contents doubles up as an index, although some would prefer a "real" index at the end of the book.
One thing to note is that many units from the rosters have from three to five special rules, besides special items that also modify gameplay. This means that they require more attention from players, but also increase the strategy involved in warband construction. Players new to miniature wargaming or those who intend to play with their children, for instance (as I have seen in some battle reports and blogs on the 'net) may find Song of Blades and Heroes a better starting point. On the other hand, the book does include rules for point-based creation of new units, so it is possible to create simpler units if one wishes (or adapt the game to other settings).
So, to sum it all up, Shadowsea feels, to me, like a more sophisticated Song of Blades and Heroes created with a specific setting. To me, it was a worthy purchase.