Read this in one sitting yesterday and really enjoyed it.
I am a big fan of the genre and of the Parker character in Richard Stark's / Donald E Westlake's novels. The tone of Darwyn Cooke's graphic novels is a little different. He seems to play with the atmosphere and it always comes off as lighter in these comics.
Maybe it's Cooke's style that gives then a cartoony feel. They seem a little less gritty and seem to have more humour than the novels and to me this holds back the building tension that is a big part of the books.
But taken on their own they are great comics.