I ate at Martin's recently, the one in Mt. Juliet (there are a few and this is not the original location). It was OK, but frankly I was hoping it'd be a little better. Definitely not bad though - I'd go back.
People seem to like Peg Leg Porkers, but I haven't been so I can't say.
I like Puckett's a lot (a couple/few locations). Not necessarily a BBQ joint, but a restaurant that serves BBQ among other things. Farkin delicious ribs and pulled pork. They have a lot of other great stuff there too.