I didn't adore this book - it lacked a bit of something I can't really put my finger on (depth? character depth?) but it was an entertaining, fun children's fantasy adventure.
Of course when I say "children's", I have to pause; I'm not really entirely sure whether it's meant to be a children's book or YA - the protagonists ranged from eight to fifteen, and while it mostly seemed to be aimed at younger kids, I got the feeling it tried consciously to go for crossover appeal with shoehorning in some wannabe teen romance (while being very cautious about it) and I'm not sure it was an altogether successful attempt.
Anyway. For younger readers (those not too squeamish anyway), I think this would be awesome fun - there are mediaeval savage warriors, there are giants, there are pirates, there is fighting, there are life-threatening situations, there is magic...
For older readers, I did feel something was missing. At ~550 pages, even if rather large-print pages, this was a reasonably long book, so even though it's only the first in a series, I'd have been happier with a bit more character development. There was an attempt at it, true, but to me it fell a bit flat (probably not helped by the fact that I never really warmed to any of the kids - they were okay, but not gripping).
That said, there's clearly a larger plot at play, and I'm rather looking forward to seeing where that goes - and I've read enough series where the first book just sets the stage with a rather straightforward adventure and the rest then take some time to develop into something with much more depth, later on. I'm not incredibly sure this is going to happen here, but it was entertaining enough that I'm happy to read on, whenever the next book comes out.
The next Harry Potter this isn't, in any case; I'm pretty sure of that.