A few days ago I've purchased Sony Reader (PRS-505). Summarizing its pros and cons I can say that's a great device.
In details. First of all it's small enough to fit in my pants' pocket. It's not tiny and almost as big as DVD-box. So the screen is of a good size to read.
Technology used to render visuals on a screen is something as innovative as paper was. As it doesn't emmit any light it provides good reading experience. Classical displays' type of light source irritates my eyes and that is the main reason I've always preferred 'hard' books to ebooks. Now when light problem is solved I can read as much as I want from ebooks.
Design of a gadget is somewhat great. It looks like a classic note book with leather cover. UI is pretty simple and there are paging buttons for both right and left hands.
Provided software is easy to use and have much in common with iTunes but enhanced slightly.
There are however some mistakes and problems. One of them: there's no buttons' locker and device must be turned off so that numeric buttons would not be pushed incidentely in a bag or a pocket.
Next. When picture is zoomed clicking 'right' on a selector turns out to slide to next image and not the right side of a picture.
There are some issues that devices hangs and needs to be reset after some synchronizations if you tried to access library too fast after a moment of disconnection.
At last there are hige problems with ebook formats. PDF files can be opened easily but they can't be zoomed and read beacuse of small font (as page is rendered completely as is in a file). This introduces a task of convertion (preferred: pdf -> doc).
Also .doc and .rtf files are rendered/formatted too slowly and this happens each time zoom scale was changed. They should better be formatted ti .lrf format (native format for Sony Readers). As direct doc -> lrf converters are hard to find, this way is preferred: doc -> fb2 -> lrf. With some manual corrections this way provides good to read formatted ebooks. One thing: standard fonts are rendered faster than custom ones.
Last thing to note: ebooks market is still very young and savage sometimes. But providing DRM mechanisms often makes things even worse.