I really enjoyed MM3. MUCH MUCH better than MM2. I agree that the encounter groups from the previous 2 books are largely useless, especially if you are a DDI subscriber. (the website has an online tool that allows you to craft encounters in a way that keeps the level right, etc.) Unlike MM2, there aren't too many wasted pages. (who needs, like, 10 different 'humans' in a monster manual?!) Also, many classics are back...Mimics, thri-kreen, lolth, gremlins, catoblepas, and so on. Some new cool additions are the catastrophic dragons (not solos, for some reason), and more psionic and primal flavored creatures.
The art is good, as usual, and the text is much richer, and better written than previous MMs for 4e. My only complaint is that they did recycle several illustrations from 3e. C'mon guys, illustrators/artists need work too.
The re-organization of the stat block bothered me at first...And id does bother me that I have all 3 MMs, and their stat blocks don't line up, so mixing encounters from all three books might be a pain. But, I do have to say, after reading through it, the new stat block layout makes more sense. Too bad they didn't think of that in the first place back in 2008. Oh well. Better late then never.
Bottom line: if you're new to DMing, and you just have the first 3 core rulebooks, skip mm2, and get this one.