Having played it a little bit, while the DLC is cool, it's a bit sparse when it comes to content, and absolutely does not justify the cost. DK Isle is not a fully fledged world, there are no bananas or fossils. It's basically a place to view statues you can unlock by spending Banandium Chips.
The new mode is fun, but it's not super in-depth, and you can only play it in 5 of the layers within the game. This all would've been great as a free post-launch update, but the value here is really being stretched thin as a $20 upcharge.