having just come off the Freedom in June, I can answer your questions-
We reserved our Supper club nights the first day. After you get on board, go up to the supper club and reserve your night. We picked a day when we were leaving port early (4PM) so that we knew we would be back on board without rushing. Make sure you don't pick a formal night (they have the schedule at the club so they can tell you which days are which). Dress code is "Smart Casual to Cruise elegant"- You will see suits and dresses, no shorts, jeans or collarless shirts allowed. Assume the dress for a fine restaurant and you are ok. I'm pretty sure the tip was not included on the bill but I could be wrong. You can ask to be sure. But you can tip what you think is appropriate.
As for what did I think- It was worth the extra money. The quality of the food was outstanding and they cook it per order, so it has not been sitting out prepared ahead of time in a warmer. We went twice during our 12 day cruise, but you should try it at least once.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007