Just as a preface, I found the previous books in the Left Behind series flawed but engaging enough to warrant reading. For a lengthy and relatively faithful rendition of how Revelation might play out in our world, I thought the books were suitable--though I would love to see this story retold more realistically and not in a preaching-to-the-choir sort of way.
All that said, Glorious Appearing was a large disappointment for a variety of reasons, most of which are problems in the writing rather than the content of the source material. The first major problem is that there is zero dramatic tension once Jesus shows up. You know the good guys won't get touched and you know all the bad guys will get what's coming to them. You also know exactly what's going to happen, because all the characters have been studying the Scriptures and talking about the prophecies which will be fulfilled. I know that the fulfillment of prophecy is very important for eschatological writing, but novels need dramatic tension to keep the reader's interest. I think it would have been much more interesting to have the focal point characters NOT always in the know, and have them struggle through these experiences without knowing all the answers before hand.
The second major problem is how Jesus and the angels speak: almost entirely in passages lifted straight from the Bible. I'd imagine Lahaye and Jenkins wanted to err on the side of caution here, not wanting to ascribe to Jesus anything that he might not say. That was a mistake for two reasons. From a dramatic standpoint, it made Jesus and the angels dull, their dialogue stale and tedious because we've heard it before (and in this very book series, too). From a theological standpoint, it's troubling because it feels like it's limiting Jesus. A better solution would be to have Jesus speak original, modern dialogue that fits with who he is (as presented in the Bible) and is tied closer to the context of the book.
The third problem is that individual perspective is all but obliterated. Very often there will be a six-page section during which the focal character's name is mentioned once... and then somehow he or she is able to witness things such as mountains splitting in two and entire cities raised hundreds of feet. This book is written like a summary rather than a personal experience. If I wanted the broad picture from a distant point of view I would read Revelation or one of the hundreds of commentaries written on that book. When I'm reading a book--particularly one labeled as FICTION--I want to know and feel what the characters are going through. To this extent, I would rather the characters and myself know less about what's going to happen, so I can experience a much richer and less predictable drama.
There are other problems with this book but those are the primary ones. If you've read most of the previous books in the Left Behind series, then you should definitely read through this one just to complete the story. If you haven't read any of the series or you've only read a book or two, you might want to reconsider before you invest your time in 12 books with a disappointing payoff.
The books are based on worthwhile material (Revelation--which I'd recommend over these books any day) and the authors seem to have good intentions (though I question why they had to stretch this out over so many books if not for money-making reasons), but the execution really falls short in the end. Hopefully the financial success of this series will pave the way for another, better fictional rendering of Revelation.