You mean as a spectator? I've only been as a performer, because I wanted to see how a couple of my songs did in front of an audience, and I thought it was great! They have a "no ballads" rule, which keeps the evening light and fun, and Jim is a great host, very funny and affable.
I'm not sure what the cost is to be an audience member, but I know Birdland can be a tad expensive. Overall, though, it's a good way to see a wide variety of music and singers, if you're not in the mood to see a specific show. The talent can be hit-or-miss, because anyone can get up and sing, so don't expect all of them to blow you away. But the bar is pretty high in NYC, so you're bound to see some incredible up-and-comers and maybe even someone who's already made a name for themselves!