I just got it this afternoon, so I'm really still playing with it. It has a slim minimalist design that I like. All the ports and slots are on the lower edge (volume buttons, headphone jack, power button, USB, SD card slot). The other 3 edges are beveled and there's nothing on the back (aside from the logo). There's an optical mouse on the right side of the screen and 4 touch buttons on the left side of the screen.
I have to say that I'm really not impressed. Onyx has come up with a number of very nice 6" premium ebook readers. They set a high standard that the M90 doesn't meet.
It's slow. And I don't mean that it doesn't turn pages as fast as LCD; I mean that it's circa 2009 slow. Considering that I can buy an iPad for less than the M90, the laggy response is inexcusable. I'm also not all that overwhelmed by the software. Yes, it does have a lot of annotation options, but I don't get the impression that Onyx customized the firmware as much as they had for the Onyx Boox 60.
In any case, I plan to write a longer review some time next week; this post is just to show off the hardware details and give me a chance to tell you what the impression right out of the box.
You can buy M90 from Onyx; retail is 379 euros.