I don't think I've ever used the dictionary lookup while reading (of course I tested the functionality after purchasing my ereader). That doesn't mean that I haven't come across words whose meaning I do not know, because I have. But usually, contextual clues and sheer bull-headedness on my part have allowed me to muscle onward without pausing.
I may keep the word in my head and look it up later if I was unsure that my contextual assumption did the word justice. But otherwise... *shrugs*
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