There's really not a whole lot about the Nitty Gritty that needs to be said other than this: It's OK at best.
The food isn't amazing, the drinks are mediocre, and the atmosphere is loud and generally overcrowded. It's nothing to write home about, but when it's your birthday--why not? Free stuff isn't a bad reason to come here, but there's no reason to come here otherwise when there's much better places to go.