Powerful, compelling writing. I was stunned by how deftly Lockhart used white space, fragments, and caps to convey emotion. Brilliant use of nonstandard effects that never disrupted the prose. It's a wildly compelling story, too-- chilling and truthful.
I had some very small issues with the way the ending worked, and how the "liar" element played out, and I wouldn't even care about those except I loved the book so much that I wanted it to be perfect. It's brilliant storytelling, and definitely, absolutely, immediately go read it. It's brilliant storytelling and genius writing.