In all probability, people who read the first of the Death Gate Cycle books and like it, are going to want to read all of the books in the series (regardless of any flucutation in the individual books' ratings). Ditto for the reverse: if people don't like the first book, then they're probably not going to want to read any of the other books in the series. Thus, instead of individual book ratings, I've rated the whole series and given a short blurb on any specifics for each book.
Overall, this is a very good series. The authors were amazingly creative in coming up with this concept and the books are well-written, extremely interesting, and internally consistent (mostly -- see below for some exceptions). I highly recommend this series of books for everyone. My comments for the indidual books follow:
Vol. 1 -- Dragon Wing: This book covers the exploration of Arianus: the realm of Air. It introduces us to the two main characters (Haplo and Alfred) and many of the recurring characters throughout the series. It's a well-paced, well-written book that's an excellent reresentation of what to expect in most of the rest of the series. Essentially, if you like this book, you'll like the other six books. If you don't like this book, then there's no sense in reading any of the others.
Vol. 2 -- Elven Star: This book covers the exploration of Pryan: the realm of Fire. Alfred is entirely missing from this book, and Haplo isn't really the centerpiece. Instead, the book is mostly written from the perspective of the mensch and of the newly introduced (and recurring) Zifnab. The one possible weakness in the book is that Weis/Hickman wrote it such that the structure of Pryan itself is more of a mystery than Arianus was in the first book.
Vol. 3 -- Fire Sea: This book covers the exploration of Abarrach: the realm of Stone. Once again, both Haplo and Alfred are present. The characters and areas presented in this book are key elements for the entire series. An interesting book, but not a very happy one at all.
Vol. 4 -- Serpent Mage: This book covers the exploration of Chelestra: the realm of Water. Both Alfred and Haplo are on this world. But, for the most part, they follow separate plot lines. There's a lot of action taking place throughout the book and Weis/Hickman introduce some actual Sartan (besides Alfred). They also bring in an ancient evil to balance the ancient good introduced in the previous book. The most irritating thing about this book is that it ends in a cliff-hanger. The previous books tied up their worlds' activities fairly nicely. Not so in this book. It ends at a very inconvenient spot.
Vol. 5 -- Hand of Chaos: This book returns Haplo to Arianus and the people he met there earlier. Whereas the first four books of the series cover the exploration of the four realms, this book starts the synthesis of the various threads from those books into a movement towards closure. It's a good book, but, unfortunately, it doesn't do anything with Alfred. The previous book left Alfred heading towards a bad end. This book doesn't do anything with that. So, while reading the book, you constantly have this little voice in the back of your mind going "what's happening with Alfred?" Irritating.
Vol. 6 -- Into the Labyrinth: This book ranges across just about all of the worlds of the Death Gate. It brings all of the various threads from the other books to a head. Most importantly, it finally does something with Alfred. The unfortunate thing about this book is that it contains a lot of technical errors (or, more specifically, contradictions). For instance, in Dragon Wing, we're told that a person needs to be familiar with an area to use a transportation spell to get to that area. Yet, very close to the beginning of the book, one of the characters uses such a spell without ever having been to her target location. There are also several instances where the characters COULD have used such a spell, but don't. Likewise, there are problems with replication spells. In Fire Sea, Haplo and Alfred use such a spell to replicate enough food for a large group of people. Yet, in this book, no one seems to consider using a replication spell to produce more of some food. But, later on, those same people are using that same spell to replicate weapons. Plus, at one point, Xar uses a spell to just materialize both food and drink for a party. But, if such a thing is possible, why do the Patryns hunt and, apparently, farm in the Labyrinth? Also, the size of the Labyrinth seems wrong. From the previous books, it's taken hundreds (if not thousands) of years for various Patryns to traverse the hundreds of Gates in the Labryrinth. Yet, in this book, from the very beginning of the Labyrinth they can see the results of things happening at the Final Gate. There's also the problem of how people at the center of the Labyrinth would know anything about the Final Gate.
Vol. 7 -- Seventh Gate: This is the conclusion of the Death Gate series. Unforunately, I didn't much care for it. Basically, it comes across as being pretty sappy. All of a sudden, Haplo and Alfred are best buds and want to teach the world to sing in perfect harmony. Most of the various threads are tied up adequately (if not very satifyingly). But, the sudden push towards liberal "peace, love, and harmony" is trite. Of the seven books in this series, this one is the least good. Once again, I didn't much care for the tone of this book. But, since it adequatly ties up most everything that needed tying up, I'm not going to downgrade the whole series because of that.