The sightlines are pretty bad if you are in the upper deck. My buddy and I sat in right field in section 502 and our view of the right field corner was blocked by section 501. If there is a ball hit into the corner of the ballpark, you have to wait for the replay to see what happened because you can't see it live. If you like to walk around the ballpark and stand for a few pitches, Citi Field is pretty hard to do that at. The upper decks hung very low and unless you kneel down, your view of the field will also be obstructed.
I do love many of the park's features though, including the Shake Shack, the Jackie Robinson Rotunda and the old Shea apple outside of the ballpark. I like Yankee Stadium and Citizens Bank Park too, very thankful to have three MLB ballparks in a two hour radius of my home.