Had tickets for 421 during the Broncos game back in October in what may have been the most exciting game in the stadium's short history. Manning vs. Romo. The atmosphere was amazing, there were a good bit of Broncos fans in attendance too, all with Manning jerseys on.
421 is very far from the field. Very very far and in the far upper corner of AT&T Stadium. The giant scoreboard definitely came in handy but even seats on the mezzanine (300s) will vastly improve your view and experience at the game. I haven't found the best source for Cowboys tickets. They're all expensive, doesn't matter if the Cowboys are playing the Jaguars or the NY Giants. Sit where you can afford but I wouldn't advise on spending a lot of money on section 421 or you'll be disappointed. The Party Pass area would even be better than these seats.