I've found it quite useful as a DM, especially if short on planning time, especially with the latest rules in use. I particularly like the short adventures, maps of mystery, and featured monsters. It's more of a DM's tool compared to Dragon.
I'm letting my subscriptions to Dungeon and Dragon lapse. (Dragon has already lapsed).
Signs and Portents is a much better magazine. It is actually giving me new ideas and inspirations, something the other two magazines haven't done for a while.