I am ridiculously anal about the way my library looks in Calibre. Covers can drive me to the brink of insanity. If it's a series, the covers must match. If the cover is hideous, I have to track down the nicest commercial one. If no decent commercial one exists, I'l create one. I've gone so far as to delete books on the fly from my Reader because of cover art shenanigans. And I'd do it again.