I can't help you because I have not sailed on the Holiday. Maybe someone else has recently.
But I can say that she is an older ship. And with any old ship there are going to be issues. Unfortunately its those issues that get the press as most people (with the exception of those on this board) only write about problems and not about the good vacations. So all those stories you have heard may make it seem like you are going to have a bad time, but don't put too much into them.
ANY cruise is going to have a group of people that have problems (real or otherwise) and who will write all kinds of nasty things as way to get "revenge" on the cruiseline. You get the same things on a land vacation, but because they are cruising, somehow people expect nothing to go wrong.
You will have a good time if you want to. Cruising is a hoot. If you experience a problem, you have 2 choices, you can do what you can about it and continue to have fun. or you can let it ruin your vacation.. its up to you. Let Carnival try to resolve any problems. If you have major problems with your cabin, ask them if they have any others open and move...
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, Celebrity Silhouette 2013