Another vote for the Cybook as a general ebook device. However, I only use it for fiction and calibre newsfeeds. Can't say how it will work with pdf's but I certainly haven't heard that it is any worse than Sony for that albeit different in the way it handles it.
No one has commented yet about the potential format change though. I'm not sure I am completely up to speed but you should look into whether the Cybook currently supports mobipocket (which you have indicated a preference for) or epub drm which is the new up and coming format I believe. I had seen a few postings to say that they were abandoning mobi on the device going forward and picking up epub instead which may be a plus or a minus in your consideration.
Finally, you may want to consider the Aztak because I think it supports mobi drm doesn't it (correct me if I'm wrong because I saw mixed responses on this)? This is the one sold at Bed, Bath and Beyond in the states and while you will have to pay exchange and shipping I think it will likely still come out cheaper than the Cybook.