It's not stable. I've managed to crash it a couple times now, and I mean I crashed it. It kicked me out to the desktop and I know that the app had to reboot at least once. It's also laggy. For example, there's a noticeable lag between pressing the home button and going to the home menu.
The app is currently limited to only the Kno ebookstore at the moment. All the ebooks are PDFs, and none of the samples have any fascinating new features. It's unfortunate, but not just because they won't let you use your own ebooks. You also can't go get free legal ebooks from anywhere. For example, if you need a copy of Shakespeare, the app won't let you get it from Project Gutenberg.
It doesn't have much in the way of features just yet. You can zoom in on the ebooks, but the zoom is not persistent nor does it doesn't support reflowing the text. You can highlight, add a text note, or add a bookmark, but you can't scribble a note. You also can't copy and paste. A clipboard function would be useful, because while you can share on Facebook or twitter, but you can't actually tweet an excerpt. It has to be typed in.
This app really doesn't have much to recommend it. I know that Kno have been working on their software for well over a year, and they've put a lot of time in developing the UX. But I can't see any of that in this app.
At this point it's just a PDF reader, and not a terribly good one.
Update: My contact with Kno pointed me at a biology textbook which showed off the app better. Kno sold me the textbook for $15 more than the paper edition (like new condition, half.com). I had a dickens of a time getting the app to register with my account, and eventually I had to kill and restart the app.
The textbook didn't show me anything I hadn't seen before. This is just a PDF reader.