I watched City of Men once before I had seen the 19-episode TV series of the same name (City of Men) and after I had watched the TV series. The movie is literally one hundred times more powerful and exciting if you have watched the show and understand the characters. If you have not watched the City of Men TV show, my recommendation is to buy that before you watch this movie and if you have watched the show, this is an amazing follow up to the series that wraps things up very nicely and is for all intents and purposes an excellent final 'episode'.
Don't buy this expecting a sequel to City of God because they are not in the same league as far as production value and entertainment are concerned. The pacing is much slower and there is certainly less action, but this movie assumes that you already know what happened in the sereies. There are a healthy dose of flashbacks that take place throughout, but these are more to remind the viewer of moments that took place throughout the show than they are to catch new viewers up to speed. In closing, I highly recommend City of Men the movie, but only after you have seen the TV show, otherwise it will still be an entertaining film, but it was made to be coupled with the series.