I also didn't think I'd use the dictionary look up as often as I am. However, often I have a good idea what a word means but I want to delve into it a bit deeper to make sure I really understand the connotations involved. I can do that now without interrupting the flow of my reading as much as going to grab my dictionary. What I now find myself wishing I could do more easily is grab a phrase and search on it. It would be nice to have multiple dictionaries including one of the references of allusions and quotations but I'd settle for a quick way to google it.
I would love to have more dictionary options, but I haven't tried purchasing a new one yet. I'm waiting to see how Amazon reacts to people loading Mobi DRM books before investing. The included dictionary is pretty good. I've rarely hit words it doesn't have. I do find myself sometimes wanting a bit more detail, though.