I think that nobody really knows about the Kindle now, because it hasn't been WIDELY advertised, only on amazon. But when people start to find out about it, I think it will be a big hit. I don't know about Sony reader, I haven't heard of it. I think this will be popular with students and teachers alike.
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I haven't used one as yet. The thing is, most songs recorded for radio play are just 3 minutes in length, so it's possible to digest an entire song, and in fact, several songs, while riding the bus, etc and at other intervals sandwiched in during the work day.
Books require more vested attention.