I enjoyed the book and it was great to get a full refresher on the long-gone browser game, but the continuity was a bit wrong (Castle of Dreams was never an available world prior to Union Cross) and it was very annoying that the protagonist was explicitly male. I read it in Japanese, so the pronouns were usually omitted, but then every so often I'd just get a reminder that imagining my player over the course of the book is wrong because novel canon says that the khx player is a boy. :( They could fix this very easily in English, but it doesn't sound like they chose to do so.
There isn't a lot of new content in this if you've played the game(s), but there is some, and some scenes are really enhanced by the choice of narrator in each. Definitely worth a read if you're a fan of the series.