Of course, there are some concerns but they have nothing to do with the Cloud Reader itself but rather how Amazon is slowly but surely locking their users into their content world. Aside from this though, this is an admirable effort in really putting new technologies to work.
I just tried out the new Amazon’s Cloud Reader (https://reader.amazon.com) built entirely on HTML5. I have to say, I’m quite impressed. All of your items are available on the Cloud Reader, save for the new lending program; that still has to be accessed via a Kindle device. But aside from that, everything else is there. The reading experience is very pleasant and more importantly, all of your bookmarks are carried between devices. They definitely have a winner here and a nice way to skirt Apple’s restrictions for in app purchases.