I have played with mobipocket, but I find the physical device not a good platform for reading. I have the Sony PRS 600 which I use for reading books now.
My BlackBerry is for staying in touch with the wide world. I read to escape. Maybe I`m old fashioned but I like the demarkation.
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My other Blackberry is a PlayBook.
I too find the BB screen too small for any real reading -- even WSJ articles can be a bit of a stretch -- but for a joke book I could manage it on the the BB and would use thee Kindle app for this.