Pat O'Brien's was our favorite, and we went several times. We were never there till after 8 or 9, and didn't see any kids at all (we were there during Spring Break).
Basically, you have two piano players. One takes the lead for one song, and the other backs him/her, and then they switch up. They have a tip cup, and you can write your requests on a napkin and leave it on the piano. If it's busy, and you leave a tip, you have a better chance of hearing your request. They'll play whatever they know, and yes, people do sing along. It can be lots of fun with the right crowd and the right piano players! It's a great place to bring someone for their birthday or special occassion because the players acknowledge those things to the whole crowd.
Of course, some players were better than others, and sometimes one would take a break, leaving only one piano player, and then it wouldn't be "dueling." Our favorite was a guy named George. There was some older lady with long gray hair that she pulled back into a pony tail who wasn't as good, and if it was just her by herself, we didn't hang out at the piano bar.
We had hamburgers there one night, and they were good. You can use your 1/2 price gift certificates just for your Hurricanes too, if you want.

It doesn't matter if you just have drinks. They're very gracious about accepting the gift certificates. Had I known, I'd have ordered extras since we wound up there about 4 or so nights!