Shake Shack IS good. For all the talk about In-N-Out burgers which have yet to see the light of day on the East Coast, I understand these burgers are comparable to those burgers. I will admit I have the advantage of being able to look out of an apartment (not mine) situated by the park, so I can see when the line grows shorter. That's when I strike.
There are other offerings on the menu, but it would be a sin to leave without at least one burger on your tray. There is also a special Shack Sauce, which is a mysterious blend of ketchup, mayonnaise, relish, and some other condiments. This is good stuff slathered on your bun - I happily purchase several at a time to keep at home in my fridge.
Also a nice touch: the Poochini, a custard treat and biscuit combo for dogs, of which there are plenty - given the dog run a stone's throw away in Madison Square Park.