I skimmed both the DM's Guide and the Monster Manual today. I've been really busy and I don't think I'm going to give either one a really good read, although I read one or two chapters of the DMG in full.
With regard to both books, I think they're great improvements over the previous editions. The DM's Guide is very much like the 3E "Dungeon Master's Guide II," which really blew me away when I first saw it. It's sort of like the Pragmatic Programmer for Dungeon Masters. It's all practical advice for how to help ensure that everyone has a good time. It struck me as so much better than the 3E DMG that I went back and compared them. The 3E DMG has much of the same content, but it also seems to have far more crap like tables, references, and rules for things that will rarely come up. I don't mind having rules for designing new classes, races, and magic. I just think that's quite secondary to common things like, "how do I understand what my players really want?"
The simplification of powers and levels really shines in the DMG and MM. Traps are just things that have powers. Monsters' abilites are expressed in terms of powers. The challenge of a given trap or monster is rated in XP, and there are simple ways to calculate the challenge of an encounter, and to tweak it. These were present in previous editions, but 4E really seems to get it down to a science. I'm sure it could be further refined, but they've gotten it to a really good point already, now.
I'm getting pretty excited about the prospect of running a 4E game.
I'm also starting to resent DND Insider, the new online services, before they've even launched. They all require Win32. Also, if they would launch a JSON (or XML!) web service providing references on rules, it would be pretty cool (and possibly easy) to write a character reference system that could tell you, "Oh, if you're checking Bluff, here are all of the characters possible modifiers."
Anyway, I'm digressing.
My final summary of the DND 4E core rules is this: they're great. I'm very happy with them, despite some minor quibbles and one or two major irritations. (Seriously, where is Lawful evil?)