I ate there last night and absolutely loved it -- fun, friendly waitstaff, very good moussaka, really pleasant old jazz, and great half-price wine (mon and tues nights). I'd much rather eat at the Hill diner than any regular diner.
I did once. Thought it was pretty lame. I guess my problem was that the word "Diner" in the name threw me off. It's definitely a cafe and no more. Don't expect an expansive "diner" menu, or too much of anything that the word "diner" entails. Go to Juniors.