Monday evening a few weeks ago at JFK T4 around 6:30pm, only a few people in line to be processed in (but only one check-in agent so it took a few minutes).
Thursday afternoon at DEN around 2pm, maybe 5 parties ahead in line with 2-3 agents, also took a few minutes.
Both clubs were quite busy but there were seats available. Much better variety of food at DEN vs. JFK (and in fact I left the JFK club for the DL SkyClub after a quick round at the buffet). Just a few minutes wait to get in there too.

(I was flying UA back, otherwise I'd just use the DL SkyClub at DEN).