Mystic River is a dark tale that has a nice mystery tied till the end.
It's one of the very few movies that sheds light on the fact that how tedious, time sucker but important detective work and it's not all jolly as shown in other movies and TV shows.
Sean Penn does a great job and you should definitely watch the movie for him.
Even the climax is thoughtful but a little damaged as well.
Though on the other side, the movie is slow paced. It has its fair share of nail biting moments that keeps you hooked here and there but still it's slow paced. Could have been 15-20 minutes shorter.
Do watch it if you want to experience the fine acting of Sean Penn and a nice overall presentation.
However if you want to watch a nice entertaining movie in the weekend, give this one a pass!