Dirty cops have been around since the earliest movies of the 30's and 50's so I'am not that impressed with his work.
Seeing as well that his character is so similar to the Dennis Franz character it's laughable to call "The Shield" groundbreaking. The character he plays is like an evil version of Franz's.
He's a decent actor but overrated. There's no depth to Mackey. He's a dirty cop masquerading as a father (he's had several women while allegedly loving his wife), he's killed suspect broken laws. The character is much more one dimensional and in any other show would have been killed of a long, long time ago. His resume outside of "The Shield" is not that impressive either outiside of "The Commish". Dennis Franz will always be the first really tough cop on tv with Chiklis following in his footsteps.