Edit: Ouch, looks like quite a few people here do. Do y'all really think that's the best way to effect social and political change? A minority of Americans screaming louder and using terrorism to vent their spleens won't solve the issue. In a democracy, everyone must focus on getting more people to vote for different policies to effect change.
In this particular case, I do find some of the violence justified, and I think it has more effect than a simple non-violent protest would. We get to see exactly how police respond when a few thousand dollars worth of property is being damaged versus when they have someone's life in their hands. The police are digging themselves into a hole here, and it's glorious.
That said, we aren't at the point where violence against people is a legitimate form of protest. In self defense, sure, but protesters should not, at this point, be attacking people. Burn down all the Targets, police stations, and cop cars you want. Let it be the cops who do all the violence against people. That's what's actually going to get peoples' attention.
As I said, violence against people is simply not acceptable here. Showing how violence is often the first resort of police rather than a last resort is the entire point of these protests.