I'll give you the downsides first. Our seats are packed a little tightly which I have come to find is standard for baseball stadiums on the top deck. Prepare to get comfy with your neighbor.
It's also a chore to leave your seats and get to the restroom. The entire row has to stand up when someone needs out.

The pros? The view of the ballpark and surrounding area is what makes 321 great. Not only is it near home plate but also on the third base side, you can see the beautiful Allegheny combined with downtown Pitt and the Clemente bridge in the distance. You take the downtown backdrop away from PNC Park and it loses a lot of it's luster.

The game seems much slower from the upper level (officially called the Grandstand) so if you can afford lower level seats, buy those. But a lot of fans just buy grandstand tickets and sit on the lowers. The Pirates have some of the cheapest tickets in baseball which I love because Steelers and Penguins tickets are outrageously priced.

Overall, I would definitely sit here again. This ballpark reminds me a lot of Comerica Park in Detroit but only better. Have fun at the game!