Really good. The book is set in 2017, and explores what might occur as phones turn into serious computing platforms and MMOs take over the world. It's also good reading stories about hacking computer games by someone who actually understands how this would work.
There's a really odd point-of-view thing, where Stross describes what the characters are thinking with a structure like "So you do this and that." I'm not really sure what he was aiming for with it, but it jarred for me (especially as there are three different point of view characters).