This book is good. Really good. It shows what the Open Gaming License can do.
The monsters in this book are for the most part creative and interesting. Swords and Sorcery Studios obviously put some thought into a lot of the monsters. True, there are monsters in the book that lack the pizazz that you would want in a campaign of heroism that Dungeons and Dragons is suited to. Yet with 300 monsters or so, it would be irrational to expect them all to be perfect.
I have to complaints with the book, each of which cost the Creature Collection II a star.
My first gripe is a personal one, but I think it's significant. The CC2 is in black and white. While this is usually issue, I feel that in role-playing, hardcover books should come in color. You may disagree, and if you're happy with black and white, then consider this book a four star supplement.
Gripe two is more significant. Whoever came up with the challenge ratings (CR) for the monsters has either never played the D20 system (...). They are way off. There are monsters in there with CRs of 5 who I would pit my not-so-buff group of 1st level PCs against without any fear for their lives. On the other hand, there are monsters who have Crs way lower than they deserve. Example: The "Time Killer" has a CR of one, yet can use a touch attack to send players ahead in time, thus seperating the group, and it's touch attack also ages players 1d10 years. It also has a sizable number of hit points. I would have reservations pitting 4th level PCs against this monster.
Two notes that are neither pros nor cons: The book is a bit campaign specific (for the SSS campaign "Scarred Lands"), but with minimal work the DM can change the monsters to any campaign. Also, this book is very dark. Lots of undead and grisly aberrations. You could call them demons or devils and have all of the Planes fleshed out with evil. If you want some scary monsters, here they are.
Overall, good, but not the MM.