Who wants confusing paragraphs you have to reread to get the sense of, or dialogue where it's not clear who is speaking, or passages where you are not certain what is going on? Good prose becomes invisible, and never stands between the reader and the story.
I spend a lot of time going over my books as I write, tweaking and improving, adding bits and changing words, thinking about the characters. As well as clarity, this adds depth and detail that makes for a more rewarding read. I'm doing it now with Ice Diaries, the WIP. (I know some schools of thought maintain an author should plough straight through the first draft, never looking back until it is complete; but in my opinion the only rules worth abiding by are those for spelling, punctuation and grammar. Otherwise, I believe in doing whatever works for you.)
So I don't get irked when a reader races through one of my books and assumes it was as easy for me to write as for her to read. I take it as a huge compliment.