I went last weekend and was really pleasantly surprised! The area is larger, better themed, and more immersive than I expected. It does admittedly feel like something is "missing" with the lack of the Yoshi omnimover ride, but everything it does have (the ride, mini games, animatronics, restaurant) is a lot of fun. I was worried the ride system chosen for the Mario Kart ride would make it seem slow and boring, but I didn't feel that way at all while riding.
Overall, a great addition to Universal Studios Hollywood, and if the rumors are true with a Donkey Kong coaster expansion, that will really make it feel complete.