I'm very excited to be able to tell my students, "Join @TheOfficialACM@twitter.com for $19 and you get all of the @OReillyMedia@twitter.com books online!" I'm less excited about O'Reilly's UI: notes don't work properly and there's no pagination! Digital books should be MORE functional than paper.
On the upside, the amount of quality content you get is staggering and the basic display of text works cleanly. If you want to read, it's great. If you want to take notes or discuss with another human, not so much.
The navigation doesn't work correctly either. This does not bode well.