So basically it's a really good advice book?
I didn't buy the DMG2 at first. I read sections of it in the bookstore, but didn't purchase it.
I broke down and got it after seeing numerous traps, skill challenges, companion rules, minion rules, artifacts (Rod of Seven Parts), and some advice for quick encounter setup and such things. The Sigil section is also a nice addition, but honestly I could gather that information from the internet, even with just wikipedia.
All that I listed are IMO usable right at the table.
By way of comparison, I never used DMG1 much after the first few sessions of gaming, when I'd often refer to p.42 and encounter building chapters. After that, I only opened it for Traps and the special rules regarding mounted combat and the like.
Last weekend when I was DMing away from my apartment (unusual for me), I only brought three books in my carrybag -- the MM2, a certain module that I was pulling some ideas directly out of, and the DMG2.
Oh I also had my 4e DM Screen -- the most important thing of all.
C.I.D.