Promenade seats that overlook the infield are much better than those that overlook the outfield. That goes for any stadium so you should be happy with any seat in 509. In my opinion, these seats give you the most bang for your buck. Sections 509 thru 517 are great for Mets games.
The rows can be a little narrow. When fans in our row wanted out, we were forced to stand to accommodate them but I'm sure most ballparks have this issue. The upper deck seats are more comfortable than at Yankee stadium in my opinion and at least you won't have the white facade obstructing your view at Citi Field.

There is absolutely nothing to do around the stadium besides getting back in your car and locking the doors. They do have a McFadden's Pub behind right field that has street access but the wait is often ridiculous. You also aren't allowed to bring your own food into the building so fans are pretty much forced to buy the expensive concession items.
Tailgating is allowed but alcohol is prohibited so what's the point and who drives to anything in New York when you can just ride the 7 train from Penn Station?