Alright, I've had time to play on all the courses online now in a 16-race tournament featuring only the DLC courses:
Mute City: I just love the results screen music. Outside of that, the course is really cool and not quite as conducive to fire-hopping as I thought it would be. It still kind of is, but I can still win on it with inside drifting bikes. Also, I like how your lateral movement is increased in the air, just like how it is in the F-Zero games. Really neat nod to the original games that takes playing F-Zero to truly understand and appreciate.
Hyrule Circuit: Solid course. I haven't played many Zelda games at all, so I feel like I don't appreciate the course as much as a Zelda series fan would but the layout is really nice.
Ice Ice Outpost: Hard course to get a good time in, but man I do really well online with it. I think it's because the entire course has ice physics, which fucks over most online players who rely solely on Speed, whereas I tend to focus a bit more on Handling and Acceleration due to my racing style. What I really like about it is that you can basically take two routes around the entire course, though I have yet to see any of the benefits of taking the yellow route in most situations.
Dragon Driftway: Loved the course almost as much as I thought I would. My first Time Trial on it was god awful, but I knew then that I would love the course once I figured out how to best utilize my style of drifting. As the name of it implies, it rewards players who know how to setup sweeping turns with their drifting and punishes those who rely too much on straightaway speed. I don't see many people liking the course due to that fact, but I love it.
Excitebike Arena: And now for the complete and total opposite of Dragon Driftway. Of the 8 courses in the DLC, this is the only one I don't like. In fact, it's probably my least favorite course in the entire game rn. You get totally fucked if you get hit with items in the wrong spot because there are very few ways to make up time. Blue Shell at the base of a hill? Bye-bye first place. You probably won't get it back no matter how good you are. If you thought Mario Kart Stadium was bad with items punishing you, you haven't seen anything yet. Raced on it twice online and got 6th and 5th after being around 1st-2nd for the first lap and a half of each race, only to be taken out by items and not having a way to make up time.
Wario's Gold Mine: I like the anti-gravity section they added to the course to make the mine-cart section a bit more interesting. The area in that section of the course seems wider, but considering you're going faster as well due to anti-gravity, I think it's a fair tradeoff. The ramp on the final turn before the finish line is a nice little touch to add some risk-reward to the finish of the race. I felt it was ok in MKWii, but I really like the changes in MK8.
Yoshi Circuit: The course layout was never bad. I just feel they could have done a little.... more with it. The drifting sections are great, and there are straightaways for fire-hoppers to make up some time, making it a pretty balanced course. It just feels... meh. The update was nice, but there's no anti-gravity, underwater, or gliding. I just feel like they could have at least made a gliding or underwater section optional, like they did with Sherbet Land.
Rainbow Road (SNES): I know a lot of people are going to complain about the fact this course has now been in half of the Mario Kart games in the franchise, but it's a really simple, yet challenging course. There's straightaways for firehopping and tight corners for inside drifters to shine. There's narrow areas that can leave you vulnerable and trick areas that can really help you make up some time.