I think one of the reasons I like the series so far is that it takes a genre that I love in movies (the caper/con) and puts it in a new setting. The series so far is "low fantasy" - there are no dragons, and very little in the way of swords and sorcery. It handles genre stories the way that I like them most (for serious stories anyway): the fantasy/sci-fi setting serves only as a backdrop to advance the characters and stories.
The Lies of Locke Lamora - This book has a few hiccups along the way that belay the fact that this is the author's first published work. For example, the book bounces back and forth in time and can be a bit confusing until you realize what's happening. Luckily, this is remedied in the second book by labeling flashback chapters as such. There is also at least one point in the book where it says "Hey look! See this? It's an allegory!? See? Notice how things seem cool but really aren't? Allegory!" Luckily, this is gotten out of the way pretty early on and the rest of the story seems to move forward at a breakneck pace without falling into the trap again. This book is more like "The Sting" than anything else. The classic con that goes bad and then works out in the end. Only, well, things just keep getting bad much past the point where I thought "Ok, this is where they hit bottom and pull off the con through incredible cunning" That alone, with the unexpectedness of the last bad twists, made me a solid fan
Red Seas Under Red Skies - I saw this as less a pirate adventure and more of an examination of the nature of true friendship with a dose of an Ocean's Eleven type Caper. There is a literal Casino bank heist, that happens to get complicated with other parties getting involved thinking they're more clever than the heroes. This is where the Piracy comes in that Nathan mentions. But the true scope of the story seems to be examining how friendships can be affected by extreme events (namely, the events of the first book) and whether friends can work past the strains that puts on the relationship. This piece was done so well that it made an otherwise by-the-numbers type caper shine. Really serves to showcase the growth of the author.
There are 5 more books in this series - If the author can keep up this level of work, he may create one of my favorite series. As it is, the Lies of Locke Lamora is ranked up there among my favorite books books of all time (hmmm...that could be an interesting topic in and of itself)