Superluminal is a strong continuation of Metaplanetary. Other reviewers' comments regarding the quality of characters, plot, and writing are spot on. If you enjoy classic space opera (as we all should), you'll be grateful for Superluminal. Be aware, however, that this volume does not provide a conclusion to the story begun in Metaplanetary, merely a continuation. I wish that the publisher had been honest enough to flag clearly that this is an open-ended series, each ending in a cliffhanger rather than a climax. There's nothing wrong with that approach, either from a literary or a commercial point of view -- I just wish that publishers would note which books are self-contained and which are installments in a series.
Update, 22 Feb 2007: Sadly, there is still no word regarding the next (and, I would hope, concluding) book in this series. The first two books (Metaplanetary and Superluminal) succeeded in creating a fascinating future world, with intriguing characters and a captivating storyline. I want to know how it ends! Make no mistake, it's well worth reading these two books, even if the third never takes form. However, like many other readers, I seriously hope the author and publisher eventually get around to concluding the series and closing the story arc. I'm ready to pre-order!