I have both. I don't buy DRM-ed e-books, so mobipocket support is not a critical to me.
From a usability standpoint, both work reasonably well. The BeBook has a more "buttony" user interface, the CyBook is more early Ipod-ish. You may have a preference...
If you ever branch out beyond Mobi, the BeBook is better, hands down. Inside the world of Mobi, the CYBook is better.
I'm looking at both of them, side by side, right now. I'm afraid I don't see any "cheapness" in the BeBook. It's a little thicker and shorter than the CyBook, and the CyBook's plastic case is black instead of charcoal grey, both they are both made of the same screen and both cases are made of plastic. I guess I'm esthetically challenged.
Whichever liseuse you buy, budget for the deluxe cover. They're worth it! The CyBook cover has two really nifty magnets the hold it closed when not in use, and it cradles the liseuse better. But, and this is a big but, the CyBook loads in from the top, which means it could slide out if it got held upside down. The Bebook uses a friction strap for a closure, (not as good), and doesn't cradle the liseuse as well. But it has two advantages over the CyBook. One, it loads into the case from the left side, which means when it is closed, it can't fall out. Second the screen cover portion of the cover folds in two, with the inside having slots for 2 more SD chips built in, as well as slots for business cards and/or thin notes. Handy.