We loved Spirit of Aloha. We used our DDP credits and paid OOP for two other meals during our trip, and it turned out that we would have been a little bit ahead, maybe $25-30, if we'd paid OOP for the luau and used credits for the meals. But it all depends on where else (and what else) you're eating.
The dessert was good. I'm not sure if you can substitute it though. I'm sure if someone is allergic they could probably make arrangements for something else.