Copying this post of mine from a different thread as it is relevant here too:
I flew out of JFK earlier this month (a Monday around 4:00 PM, flight was at 7:00). I first went to the Centurion lounge, which was very full and I barely found a place to sit. Then I went to the Skyclub as it got closer to the time for my flight. There were two lines, a priority line for D1/Diamond/360 passengers, and one for everyone else, both astronomical. I was flying in D1 so I joined that line and waited for about 15 minutes to get in, which wasn't too bad. However, once I got in, there were plenty of places to sit and I've definitely been in the Skyclub when it has been more crowded. It will be nice once the new Skyclub at JFK opens up to improve capacity, but I think next time I will try the Virgin Atlantic one.