With any DRM scheme you are locked in. Sure, ePub with DRM is sold by several ebookstores but all that has to happen is for Adobe to turn off the DRM server and unless you've stripped DRM you've lost your DRMed ePub files. I've had it happen to me from both Adobe, Amazon, and Fictionwise (when one of the DRM servers stopped working through them). In only Fictionwise's case was any attempt made to provide the lost content.
Doesn't matter who provides the DRM, it's still a lock in.