All the Isles books follow the same basic pattern, and Master of the Cauldron is no exception. Anyone familiar with the series would be able to predict several major plot points before ever cracking the spine.
You think I'm kidding? Check this out: The group of friends gets separated by magical means almost immediately. (Gasp!) Cashiel wanders off on a quest to help a (magical) damsel in distress. (Shocking!) Sharina ends up sucked through space and time to face villains. (Unheard of!) Ilna is abrasive, Tenoctris foils powerful wizards whilst being self-effacing, and Garric spends a lot of time laughing and grinning at comments made by the voices in his head. (Say it ain't so!)
That said, and sarcasm aside, the author as always has provided a pleasant read. There's nothing new here, but it's a good story nonetheless, and the book is eminently readable. Unlike some other "fantasy" authors, Drake clearly has a fine appreciation for craftsmanship. He has done his homework, and the world of the Isles is rich in history and detail. Battle scenes show a sense of realism in heroes who get tired and show fear and sometimes even retreat to fight another day. If the individual strands of the plot follow familiar themes, well, if it ain't broke why fix it?
I think this book could safely be recommended to readers unfamiliar with the series; it stands alone fairly well without falling into the all-too-common trap of overwhelming the reader with backstory. If you're not quite sure who all these people are, there's still a rousing blood-and-guts adventure to keep you occupied.
For long-time readers, however, I might recommend waiting for the paperback (or check out the publisher's website for the cheaper e-version) rather than spending hardcover money on a book that, for all intents and purposes, you've seen before. But if you know what you're getting into, and if you've enjoyed Drake's work (and the Isle series in particular) in the past, you will almost certainly like this book as well.