I am a humungous fan of Brian Azzarello, and like most of his work I really enjoyed Superman: For Tommorow. Firstly, the premise of the story of one million people disappearing, including Superman's wife, Lois Lane, so Superman has to investigate this enigma was an enticing premise. Secondly, I thought the dynamic between Superman and the priest character was very well done. Finally, the storyline just feels very classically 'Superman' in a way while also having a sense of modernity and slickness to it, brought mainly by Jim Lee's artwork.
However, it seems I am in somewhat of the minority. Wikipedia states the storyline received 'mixed' reviews, and I see a lot of scorn and criticism towards this storyline. Why is this? It's not perfect, however I would hardly call "For Tommorow" bad, let alone mediocre.