Have to give a dissenting opinion...it's only good if you like hot bananas with a slimey texture in a sickeningly sweet fried bread (just my opinion, lol. But I do suggest you try it before adding any syrup: it really is VERY sweet)
we could barely eat one slice of tonga toast split between 3 adults. Just was way too sweet and not a nice texture inside.
But if you're determined to try it, go to Captain Cook's counter service in the Poly. It's cheaper than getting it at Kona.
One of the non-Disney restaurants...think it's Rainforest Cafe over at Animal Kingdom...has "tonga toast" on their menu but it's not the same thing at all (bananas are not stuffed inside the french toast, just served on top)