I hate the 3.x DMs guide.
At first, I loved it because it had all sorts of "Behind the Curtain" tips, charts for rolling up cities, random treasure, pre-generated NPCs, etc, etc.
But then I started designing and running adventures. You know, what DMs are supposed to do. I grew to loath the DMs Guide. Why?
Everything you NEED is scattered across 200 pages and hidden in little charts. ECL? Character Wealth? Challenge Ratings and Treasure... Gah! Now somebody's poisoned... you have to be kidding me, where is all this stuff?
I even got pissed looking through the 3.5 MM recently. I was looking at the Derro entry... spell like abilities AND poison, with no stats listed. Great, now I have to spread 3 books across my living room couch so I can understand exactly what these monsters do. Derros are a CR 3 monster... great, if my party runs across 1 lonesome Derro... but how many would be a challenge against a level 5 group?
Of course, there are easy answers to all of these complaints, but you have to flip through multiple books and multiple places within those books to get a complete picture of what's going on.
I can't WAIT to see what the new DMG has and I'm praying that it's put together more logically.