Wow, so many responses in the thread that just say "feels like normal rumble." That's fascinating to me because I felt the difference right away with the pro controller. Regular rumble just sort of buzzes from the center of the controller (or that's what it feels like). The rumble in Mario Kart 8 creeps up and down the grips depending on what you're doing. Reving up the engine at the beginning of the race is especially noticeable.
It's not a huge deal, but it fits the sound design well and feels nice. Good addition.