It is what you make of it.......I've taken mine at 9mos, 13mos, and 18mos. The 9mos was cool to watch him notice everything, and we got some nice Christmas photos of him. My second son went at 13mos, and he didn't fear anything. He took in everything, but his favorite was the ducks visiting our patio in the mornings and he got to feed them. At 18mos, he was fun to watch, and to hear him yell "MicMou". But........he was now at the age of being afraid of Pirates, and Haunted Mansion.
My meaning of 'what you make of it' is: You have to go not expecting to do everything together. If someone in your party is going to be bummed because accommadations have to be made; such as parent swap, you might get bummed to. My husband and I went knowing not to expect to do everything. When the boys were toddlers; we went with the attitude that we were just going to relax, and take in the sights. We got up at 7am, since the boys were at daycare for 6am anyway. We went back for naptime at 12 or 1 and they were refreshed and ready to return for 4 or so. They loved Magic Kingdom at nite. but my boys were very laid back and were the 'go with the flow' kind.
It was nice taking them so young despite having to tote the stroller, diaper bag, my bag of stuff, video camera etc.., then having to board the bus for naps, and do it all over again in the evening..........we were tired, but those memories are priceless. Now that they are 14 and 11; it was a thrill seeing them last year ride ALL of the rides, especially the ones that I won't ride anymore (we hadn't gone in since 1999).