I got my copy of the 5th Edition of the Dungeon Master's Guide the other day. I have not read through the entire book in detail yet but I have read a good deal of it.
I am prepared at this point to make the claim that this is one of the best editions of the DMG ever.
It stands up extremely well to the classic 1st Edition DMG. It is hard to match the 1st Ed DMG in terms of content, but this new book does an extremely good job of coming close.
It is certainly better than the 2nd and 4th Ed versions. It has an edge over the 3rd Ed version as well, but that is only because at the moment I know the 3rd ed one better and the 5th ed one seems to have more material.
Of so far it seems that 1e, 3e, and 5e are the stand outs.
What strikes me the most about the new 5th ed DMG is that it is full of options, not rules. It gives DMs ideas on how to make rulings, much like old-school games but leaves the details up to the DM.
Honestly if you are an old-school DM and you can't find something of use in this book then you are being unnecessarily obstinate.
This book continues the trend of reading like a "Greatest Hits" of D&D. Everything from the earliest supplements to 4th edition is here in one form or another. Sure you won't find the table of Harlots here, but the sample dungeon (also reprinted in 3e) is here.
The Great Wheel cosmology is here along with Sigil (covering 1st and 2nd Ed) the define your own multiverse is here (3rd Ed) and places like the Feywild and Shadowfell (4e) are here. In fact this book makes all my 4e fluff books usable again. Yeah the mechanics will need to be altered in some cases, but so fare things like DC still line up real nice.
I am looking forward to spending more time with this book. I am also looking forward to going back to my 1st Ed DMG to compare and contrast.