With the plethora of DRM killer and conversion tools available, format support is not critical: you can kill the DRM and convert from one to another quite easily (a bit of a learning curve and confusing at first, but OK if you have basic computer education).
I bought a Bebook. I still find the Sony more sexy as a device, but I could not resolve to buy one purely for philosophical reasons. The way the device is means to prevent you to buy books from other sources than the Sony store. This is easily circumvented via the aforementioned tools, but still, I refuse to support this approach. So Sony won't have my money.