My concern right now is less the DRM than the content availability. Sony, for all it's bad DRM history, seems to at least have addressed this, while content for the others are a big question mark at this point. Since for me this is a recreational reading device - not for practical, educational, or business use, I would be really hesitant to buy anything but the Sony until I know that the same or better level of content will be available for the others.
I would also think that textbooks would be a wonderful niche for these. I can remember carrying many pounds of bulky texts to class though 8 years of college! I wonder if anyone is planning this use? It could be a big foot in the door and given the cost of college texts, the $350 or so cost of the devices would not be all that significant.