OK, looks like I get to post the first review of this manual here. I gave the original 3E Dungeon Master's Guide a rating of 2 because it was so poorly organized. I had to hunt around quite a bit for the information I wanted to find in it and it's organization never really lent itself to my memory, unlike the Player's Handbook which had a much better organizing principle based around the steps in building a character.
The 3.5 Dungeon Master's Guide is much improved over its predecessor. The rules most likely to be consulted during play are put near the beginning and near the end of the manual where they will be much easier to find than in just about any other place in the manual. Special Abilities, Condition Summary, and the Environment are all sections in the end-of-book Glossary where they will be easy to thumb to when you have to look up some relatively esoteric aspect of a monster's special attack and how it affects the character.
At the beginning of the book, after the basic introduction to running the game and terminology, is the chapter on actually using the rules. Again, like the expanded Glossary, this is pretty easy to thumb to when the DM needs to review a rule. And, during play, sometimes speed and avoiding halting play too long is a very important thing. Rewards has also been moved to this section of the book, including the distrubution of experience points. Previously, I think Rewards was part of a section on building adventures (I no longer have my 3E version, having already sold it at a used bookstore to defray the cost of the new book as well as pick up some Peter Gabriel on CD).
As far as new content goes, most of you have probably already seen rumors of what has changed on the boards and are aware of the changes. Nothing is particularly earth-shattering. Some magic items have changed a bit. Elven cloaks, for example, now provide a +5 bonus to Hide instead of +10. There are a ton of new trap examples as well as guidelines for constructing them within the framework of the game rules. Non-Player sample characters are actually a bit less fleshed out than they were in 3E because the skill points aren't distributed nor feats assigned. The tables, however, are considerably easier to read and gear is now placed on the table and thus easier to parse out. THere are elements of the Manual of the Planes included as a basic cosmology for the campaign. There are also a few new prestige classes in the DMG that, for the most part, debuted in other sources including Arcane Trickster, Archmage, Dragon Disciple, Duelist, Eldritch Knight, Hierophant, Horizon Walker, Mystic Theurge, Red Wizard, and Thaumaturgist. Some of these are sure to generate hot debate at your gaming table, particularly the controversial Mystic Theurge.
Overall, I give it a 4 of 5, in no small part, because it is a much more effective reference tool than the previous edition. I've had it a very short time and I'm already more proficient in finding the sections I need than I was with the previous one after 2 years of running the game.