Frank Milller is considered one of the greatest writers in all of comics. While I love Frank's work, I wouldn't say that he is as great a writer as he is an artist. Unlike Grant Morrison or Alan Moore, whose writing always shine through no matter who the penciller is, Miller's writing is so interlocked with the visuals, that his stories live or die depending on the art.
For example, let's compare the Alan Moore penned Spawn 8 and the Frank Miller penned Spawn 11, both illustrated by Todd Mcfarlane. McFarlane is a great artist, but I wouldn't say he's a great storyteller. His art, while impressive isn't always the most coherent. Regardless of this, Alan Moore wrote a great issue of Spawn. His witty humor and inspired world building flourished in spite of McFarlane not being a great match for the writing when compared to Kevin O'Neill on League of Extraordinary Gentlemen, or Dave Gibbons on Watchmen. In contrast to this Frank Miller's Spawn 11 is about the same level of quality as a pedestrian issue of Spawn: Confusing and needlesly verbose.