Boy is that ever the truth Lisa....we stayed at YC Concierge last Dec. and I can't imagine staying anywhere else.
Anyway, to help with your question Kim....It would be best to book concierge ahead of time. One reason is to take advantage of any discounts that may be available (i.e. Disney Club or AP). I got a significant discount using my AP (purchased just for the discount). Also if you are booked into concierge, you can avoid standing in a line for check in. You let the valet know you're concierge and they take you directly upstairs. Nice perk.
Now is it worth the extra money? I think so. But like Preference Hair Color.....it's because I'm worth it

I like the extra pampering and am willing to pay for it. We had DH's elderly parents with us (booked them a separate room) and concierge's help was invaluable.
Concierge also got us a Cindy's ressie for 13 people, which I thought was an impossible task.
The food in the lounge was wonderful. Breakfast offerings were just enough for us not to have to go out to breakfast. Also, the fact that they had the coffee out at 6:00 am was a big plus for me, as I'm an early riser and there was no place else to get coffee that early. In fact we were stopped frequently on our way to the hot tub (early am ritual for us) and people asked "Where did you get the coffee?" I replied as non snooty as I possible could "In the concierge lounge, sorry"
In the evenings the appetizers and desserts were terrific.
Well I hope this helps. Feel free to ask more questions.
Linda