I liked this one a lot. Long past his heartthrob era, Johnny Depp became a real actor able to deliver a chill when his face subtly changes to an expression of expectant evil -- which it does frequently in Black Mass. Telling a true story about a mobster responsible for so many deaths could become a "Valachi Papers" exercise in just one killing after another, which this film does, but there is that deep family / friendship / loyalty thing done in a fresh way to keep it real. It also mixes in a lot that is Boston as a character, we really felt the desperation of citizens, the detachment of the authorities, the rot of the bad parts of town. The story was engrossing, and as each newly introduced character lines up in either the "how's this one going to get it?" or "I'm gonna get that guy" line we followed the whole epic with interest.
You have to review a movie on what it delivers, not what it isn't, so I should stop there. How you feel afterwards, however, is fair game. We both wanted a "big set piece of action" somewhere in the second half -- something to really say this was a movie -- and also to 'feel' a bit more for the guy who, in the end, was so murderous that would probably have been inappropriate. Lastly, we did enjoy seeing the very things we had seen in the news about this guy. He was interesting and the film is interesting.