I use an Ipod Touch as my primary reader and Marvin as my primary reading app. Marvin can no longer make a direct connection to Calibre through USB (stuff broke with iOS updates) but it is possible to download books directly to Marvin using Calibre's content server.
The main disadvantage is that screen size is tiny. For me the ability to slip it into a pocket and read one-handed makes up for the screen size but it is a real disadvantage, particularly if you care about covers or your books have maps or other graphics.
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