I participated in the NYRR 5th Ave mile in 2014. It was a great experience! Since it was right before the marathon, it was nice to keep how all of my training was paying off on a very flat course straight down 5th avenue. The heats go off by gender and age group, so it's structured a bit differently than most NYRR races, but they are great at making sure everything goes off on time. You get a nice t-shirt when you pick up your bib, too.
It's a lot of open space, so you're able to sprint for a mile and not really bump into anyone, and once you finish they have water waiting.
I think it's a lot of fun.