Artwork is important, as it gives you an impression of what something is going to sound like, even before hearing any of it. If the artwork is exceptionally well done, my expectations go way up. Mastodon's Leviathon has outstanding album art, and it does make the album seem more complete, more of a finished product, as stated above. A good album cover can express the mood of an entire album, i.e. Opeth's Morningrise, Leviathan, My Dying Bride's Songs of Darkness...etc. Boring album art usually will make me expect a lot less, and not be psyched for an album i.e. Anathema's cover art.