So far the DC store's offerings are rather slim, especially when you take into account all of the titles the publisher handles. Most of the titles available are the more recent ones, such as Green Lantern: Rebirth and Frank Miller's Batman. One very awesome offering of note in the store: Neil Gaiman's Sandman; it's only one issue so far, but that's a much better start than zero.
ComiXology also has their own comics app and, just as the DC, Marvel and BOOM! apps are, it is free. Some of the same titles offered in the individual comics apps are available within the ComiXology app, though you can only get DC titles within their official app. [Update: ComiXology's official app was just updated with a specific DC Comics section.]