The original Undermountain campaign gave us a small look at the largest dungeon gamers had every seen. Monsters were everywhere, traps around every corner, all kinds of treasure (one of the subtle dangers of Undermountain was carrying too much treasure) and an adventure that just never seemed to end. This book is a 3.5 edition update of that original adventure, but it fails to capture the feel and adventure of the original.
First, one thing that anyone who saw the original boxed set remembers was the maps. They were poster sized and just huge. The Dungeon level map on page 19 is only a fraction of the original map (about one fifth of the original). The maps in this book are of lower quality and some are just wrong. The map of the Yawning Portal Inn is missing the yawning portal! The very well you use to descend into Undermountain.
Second were the rooms, each with traps, monsters and treasure. The original book had lots of new monsters and magic and the treasure was described in detail unique to the Forgotten Realms. In this book an encounter that would have taken up two paragraphs are expanded to fill two pages. There are very few new monsters (but there are many references to monsters from other D&D books), the treasure is blah and there are only a few new magic items.
One of the interesting aspects of the magic items in the original book, was that many basic items were given little quirks that made them special; A longsword +1 that was unbreakable, a dagger +1 that was invisible or chainmail armor +1 that would float. Unfortunately, 3rd edition D&D cannot do this without item creation details, creation costs and other information that the player really does not care about.
Perhaps it is the changes from 2nd to 3rd edition of D&D, but the original book did not waste so much space on behind the games details, a trap was just a trap and did not need exact technical details of its creation. In adding all this information, all the details that made the Realms special are lost. In the original the coins included those of individual nations, the gems were described by type and cut and alternate treasures such as food (we are in a deep dungeon) were included.
Overall, this book is just very disappointing. Not only for those of us looking to recapture memories of past adventures, but also foreboding of the upcoming Expedition to the Ruins of Greyhawk. I hope they put a little more effort into that one than they did into this one.