Nebulous
Legend
I think it's one of the best 4e books to date. So many good things in it.
DMG 2 is one of the best books WotC has released for 4e, and probably one of the better gaming books for gamemasters in general. I have used it extensively and have found the advice solid, creative and very inspirational. I would say it is a requirement for anyone running 4e whether you're new or old.