I've only scratched the surface so far, but it is the challenge mode it was presented to be.
Donkey Kong Island is nothing like Mushroom Kingdom in Super Mario Odyssey. It's merely a nostalgic place with the overworld music from Donkey Kong 64. There are no new bananas to collect, the only thing worth of note is that there are six golden plates to enter bonus levels that yield a lot of gold. Other than that, it serves as the introduction to the challenge mode which the DLC is all about, plus some statues you can spend your chips on.
The goal of the new game mode is to collect smaragds (a color swap for gold, really) in a set amount of time to fulfill a certain threshold to move on to the next round. The first challenge consists of six rounds, each lasting 100 seconds. Big bananas and fossils give you the option to select between three randomized upgrades to multipliers for your score (there are more than 100 of these perks), and all these upgrades reset when you start a new challenge. The system is like a roguelite game, so aside from these temporary upgrades, there are also a few permanent perks you can unlock.
The selection screen has room for up to eight challenges per world, but only the first three are available at the beginning. I do not know yet if there will be eight challenges per world. Higher level challenges go over more than six rounds and feature progressively higher thresholds you have to reach to keep yourself in the game. One challenge run will take between 15 to 30 minutes, so if there eight challenges in a world, that will take around three hours to play them all only once. By the looks of it, this challenge mode will be available for at least ten worlds in the game, so more than 20 hours of playtime is a conservative estimate for this DLC.
In the end it comes down to the question if you like this sort of gameplay, because the DLC is clearly not more of the same as in the main game. Challenges within the same world won't let you see something new, they only up the difficulty. But that is par for the course for roguelite games, you play the same thing over and over again, each time with a little bit more benefits.
If you like this kind of gameplay loop, the DLC is worth the price given the estimated length of it. If you don't like this kind of gameplay, it's easy to pass on the DLC and you won't feel that you are missing out.