City of Men is very well-made. The cinematography in particular is stunning--there are some beautiful shots of the shanty towns lit up at night that were not present in its predecessor.
But unfortunately, it lives in its predecessor's insurmountable shadow. City of God's scope was wider, its message was stronger, its characters more memorable and its child violence more disturbing. City of Men is a great movie, but it would be like making a "pretty good fantasy film" as a spiritual successor to The Lord of the Rings.
City of Men is a solid film on its own and out of filmic context, but in relation to what came before, it's merely a supplement. I started the movie expecting "not nearly as good as the first film, but still good," and that's exactly what I got. The problem with masterpiece cinema is figuring out what comes afterward.