Neither of the previewed Demon Heroes are viable yet. Although Obliveron has some powerful possilbities, it lacks Ability/Equipment destruction, (only Totem of Decay and Spiritual Domination on Tatulla) has no equipment as has been mentioned, and cannot heal/protect itself except for Sarmoth and Hukkath. However I do have a nice Obliveron deck that has been working well for me =] (see "Fanatic Obliveron" thread if curious) so with a boost from the next set it could become pretty threatening.
Basically, with current options, Demon Heroes can't compete viably, but from the next set there will most likely be some powerful builds.