MK8 reviews are coming out. They're all in the 8-10 range.
http://www.ign.com/articles/2014/05/15/mario-kart-8-review
Mario Kart 8 is the best kart racing game Nintendo has made in a long time. It strikes a careful balance between refining old ideas while introducing fresh new ones. Admittedly, its gorgeous graphics and jazzy orchestrated soundtrack bolster its presentation, but you’re getting a lot more than just looks with this one – although I spent an awful lot of time gawking at the details in slow-motion, the fast and furious pace of racing with friends both locally and online is what really kept me coming back.
http://www.gamespot.com/reviews/mario-kart-8/1900-6415760/
Racing skill determines who wins and who loses. That's true if you're competing with a group of friends (local splitscreen supports up to four players, and you can play against 11 people in the game's smooth online races) or striving for gold medals against the computer (50cc, 100cc, 150cc, and Mirror Mode are all here). And by having the importance of weapons significantly reduced, it's now much harder to make up ground once you're stuck in the middle of the pack. There's no instant-win item to come in and save you, so you have to climb back slowly and surely by maximizing your speed and finding the best racing lines while hoping those in the lead make a mistake. Defense has become more important than in previous games. With the introduction of the super horn, you can repel any item flung at you, and simply keeping a shell or banana trailing behind your kart blocks almost any attack mounted against you. Even the star has been toned down. It still grants you invulnerability, but you're no longer infused with super speedy gasoline, which lessens the impact it has. It's fascinating just how different Mario Kart 8 is from, say, its Wii counterpart. It's so much less chaotic and so much more focused that their similarities are largely cosmetic.
http://www.theverge.com/2014/5/15/5714102/mario-kart-8-review
The best 'Mario Kart' ever
Mario Kart 8 for the Wii U finally provides some hope. If I get lucky and grab the new super-horn, a box that emits a stunning pulse strong enough to knock back other racers and negate the explosive impact of the blue shell, it'll all be okay. It's a rare item and I've only collected a handful after playing hours of the game over the past two weeks, but that hope is enough. Yes, a blue shell is coming. But maybe, just maybe, I'll be ready for it.
Mario Kart 8 isn't drastically different than past games in the series. The structure, controls, and items are all largely the same as the Mario Kart games you played on your DS or Nintendo 64. But a few seemingly small additions and tweaks, like the super horn or the new anti-gravity feature that lets you drive on walls and ceilings, have a surprisingly big impact on the experience. Mario Kart 8 is full of life and ideas — and it might just be the best game in the series to date.