Plenty; both good and bad, has been written about the original Creature Collection. Most of the bad comments centered on the large number of statistical errors and some pretty sub-par artwork. The artwork never bothered me, but the stats for many of the monsters had to be seriously edited to make them usable in a game. Muffed stats torpedoed many a monster. It was obvious that the book was rushed to market in an effort to beat the first 3.0 Monster Manual to the shelves.
Despite its shortcomings; however, the Creature Collection was full of creative beasts that I have found much use for. The Creature Collection Revised keeps all of the original's monsters, but fixes the statistical errors and even has some improved artwork. As part of the revision, this book is now compatible with the 3.5 version of the D&D rules as well. If you're considering buying the original Creature Collection, skip it and get this. If you already own the original, this book is well worth the price of updating.