You do not need to make reservations--they don't even take them, except for New Years Eve possibly. There is a $12 cover charge.
Doors open at 7:00 p.m. and pianos start at 8:00. There is no need to get there at 7:00. The first hour is usually pretty empty, and the piano players are very chatty with the audience, and will play slower stuff they won't play later in the night.
The guys play requests all night long, so make sure you bring some ones and a pen to write down your requests. Most people put up a couple dollars per song, but if you put down more they will probably play your song a lot faster (one of them calls a $20 the Jellyrolls fast pass). The piano players know an amazing number of songs--I don't see them stumped that often.
I think it's totally worth going to. I've spent many nights there. The piano players are fantastic (Scotty, Ray, Mike, Tommy, and Jason are my favorites--if you go and see them, tell them Karen from Boston sent you).