i think the people who made this movie genuinely like and admire police officers. The police officers dpicted in this movie are human and in the two main characters, admirable. You really come to like and respect them as individuals. You root for them, not because they are the "good guys" fighting the "bad guys", it's more real, it's more deep than that. You feel their bond to each other, you feel their desire to protect the defenseless.
The movie lets you get to know Officers Taylor (Gyllenhaal) and Zavala (Pena) on the job and at home. There is plenty of humor to lighten up the gritty, ugly things they see in their work. Both actors are believable in their roles. Pena is as good in this as he was with Mark Wahlberg in "Shooter"
The supporting cast is good as well. The filmakers also veried the bad guys. Not every criminals are evil, some are sad and some are just funny. The film covers that whole spectrum.
Which all adds up to an ending that grips you and, in my case, brings tears to your eyes.
I liked it, can you tell? I highly recommend this movie.