I have both the Bebook and the CYbook gen 3 (no Sony).
All the formats I have tried in the Bebook are rough around the edges (although I haven't tried them all). Cybook does Mobi well, but everything else is rough around the edges (and with significantly less format support).
However, Bebook is the testbed for Openinkpot, which is making steady progress. I could be happy with any of the readers from a hardware standpoint, but all of them are too weak from a firmware standpoint.