The mobile App.
First, I use the iPad app. I've just recently loaded the android app on my phone...
I was sort of disappointed with it. While yes it's functional for "not yet released chapters" it's in my view a little hard to browse in but the major issue I have is...
You can't read books from your library direct.
You also can't "purchase" using your credits.
It'd be nice if one of those where possible.
I the other thing which would be nice is to be able to download the volume in "app native format". For instance most of my books are on kindle. I'd much rather read from this app but for some reason things go wonky when I try to load it there (even after using a conversion service).
Right now I'm using adobe digital editions to read from which isn't bad, but not exactly preferred.
I use calibre to convert the epub into mobi. You could also send it to the kindle conversion service from amazon. Maybe this would work for you better. They will sen you the file either over WiFi or with 3g over the cellular phone.
But for the first two points. I am 100% for this. Kinda bugs me that I can not read my ebooks directly from the official app. Also it is a lil rudimentar function wise.