The 3.0 d20 Monster Manual was nothing special to me, it was missing a few factors that would have made it all the more special, this book finally ammend for this and includes all of what I thought was missing from the original d20 MM. All monsters and creatures now have the following added to their profiles:
AC for flatfooted and touch attacks, saving time in calculations and 2 rates of attack, their standard attack and their full attack.
Several other things have bee naltered, such as how Damage Reduction works, I feel this change in particular is one for the better. Many more moster now contain a section that has it possible to run them as player characters. Designing and advancing monsters has been expanded greatly and is more comprehensible, also, the Skeleton and Zombie have finally been made into Templates, a great move but something that should have been in the previous MM.
Out of all the revised Core Books, I feel this is the only one that is worth purchasing, the others can be suplemented by using your existing PHB and DMG with the update documents Wizards have released on their website for free download.