I've been here for three different things including a concert and numerous Mets games. Has a lot of technology and nice layout. The escalators are confusing and lead you to all different areas of the ballpark so make sure you absolutely know where you're going. The ballpark is very fan friendly and the fan base is much more laid back than the Yankees fan base. But if the Mets were to ever get hot, I know there are millions of Met fans in NYC waiting to come out of the woodwork.

Section 136, row 4, seat 5 and 6 were where we sat exactly (girlfriend and I). She doesn't care too much for baseball but the atmosphere is definitely something she wants to return to.
We had the perfect angle and sitting so close to the outfield was a real treat. The outfield seats look and feel a little different now that the fences were moved in a little but I would still sit there again in the future. Could see all of the shifts of the infield defensively really well. No bleacher seats at Citi Field, every seat is it's own individual seat so rest easy if you're expecting that in 136. Great section to sit in for the possible souvenier home run ball.
The heavy winds are always noticeable on the upper deck so avoid those seats if the weather is windy. NYC is still on the coast so don't underestimate this.