I haven't had them in several years but my recollection is that they were somewhat similar to litchi in flavor, perhaps even slightly better than some litchi. The negative is that there is much less flesh per fruit. There is a fair amount in the upper portion, the "shoulders" of the fruit, but there is none towards the bottom of the fruit -- the seed is in direct contact with the skin with no or minimal flesh in between. I have only tried one selection (the one here at the Hilo fruit arboretum) so I don't know if they vary much in flavor or amount of flesh. I don't know it is something you would want a grove of but I have a few in a close "communal planting".
There seems to be a fair amount of confusion between alupag (Dimocarpus didyma) and the variants of the tropical longan (Dimocarpus longana var. malesiana).
John