As I fly 3 or 4 times per annum to the US on United, I'll offer some thoughts.
The last 8 flights have had the new J class seats. I avoid the front row on the top deck because it is next to the 2 toilets with the noise, smell and qeues especially after breakfast. Also, the legroom is worst here and meal and drink service last.
Row 15 has plenty of legroom but usually no space for storage under row 14.
Most importantly, the FAs often use the exit area for the parking of the food and beverage trolleys. Not good if turbulence causes one to fall onto you!!
If I do use the top deck, I'm going for the last row. Drinks come first as does the dinner which is important on the US to Australia service where, at about midnight, you really just want a quick snack and a decent glass of wine and get to sleep! There is a big baggage locker just round the corner and I usually just go down the stairs to have the choice of the 3 toilets in that J class section.
As already noted, the other big downside is the poor overhead storage space and delays in deplaning if other pax are slow descending the stairs or someone has to rummage thru the back stowage to find their oversize carry ons