I think this is a great trade for the Cowboys. First round picks are WAY overrated. Half of them (or more?) are busts anyway. Cooper is a good receiver and could be special. Like with Harvin, the problem with the trade wasn't the draft pick, it was the money. The only downside I see with Cooper is that he doesn't have much in the way of cheap years left and they'll have to decide soon if they want to keep him or not.
Raiders are clearly building for Vegas, which isn't a bad idea. It's so far off Mack and Cooper would have less in front of them than behind them by that point. Hopefully for their sake they are able to hit on draft picks (certainly not a guarantee, see above). But let's all be honest, we don't like the Raiders and want them to suck. But all of that changes once they are in Vegas. I'm not saying I'll be a Raiders fan when they move to Vegas, but I'll certainly want to see them do well so that their home games are awesome.