It's the best 4e monster book yet.
I'm thinking of picking up MM3. But, can anyone tell me about the errata? Does MM3 require modifying the monster stat blocks so that they match the current rules?
MM3 follows the current errata/rules and format.
In fact, as can be seen in this thread and others, people try/want to bring MM1 and MM2 monsters to be on par and up to date like MM3 is.
Having said that, MM1 and MM2 have some of the more iconic monsters. But you could always take another monster and reskin it as needed.