I went two years ago, and I'm pretty sure it's gotten bigger (and more expensive) so my impressions may not be all that relevant anymore. At the time though about 95% of it was an enormous retro gaming flea market. The only new thing I recall they had there was Red Octane demo-ing the original Guitar Hero before it came out.
It was pretty low-rent, but it was fun. And cheap, which was definitely not the case last year so I skipped it. But if you collect old games and systems, holy cow. I cannot think of an old console or 8-bit computer that was not available there. If I didn't have to schlep it back to NYC I would have walked out with an Atari 800.
ETA: Looking at the site, it's a reasonable price again this year. And Rock Band is there, nice! Everything else seems pretty similar to when I went.