I want to give my quick thoughts about the courses.
Yoshi Circuit: I love how they redesigned this course and it really benefits from being in HD. I've always liked Yoshi Circuit, so I am glad that it made a comeback.
Excitebike Arena: It's alright, but very faithful to what the original Excitebike was like. The random courses makes each race interesting, but I noticed that there were two variations.
Dragon Driftway: Personally, this is my favorite course from the DLC pack. I love how the track is design and I love how it looks aesthetically.
Mute City: Very faithful to the series that it comes from! It's a fun course that is filled with great designs and the course is very colorful! The F-Zero music playing before and after every race here is a nice touch. Oh, and I noticed a billboard where Captain Falcon said, "Show me your moves!"
Wario's Gold Mine: I like how the course has more of a roller-coaster feel than the original course. Other than that, I'm indifferent.
SNES Rainbow Road: I'm not a fan of this course and I turned my head when I saw it return... those feelings are still there.
Ice Ice Outpost: This is honestly my least favorite course in the DLC pack. I am not a fan of how the course is designed and I find it to be bland overall.
Hyrule Circuit: What a fantastic homage to a wonderful series! I love how the course is designed and I love all of the references to Zelda (replacing Coins with Rupees, the item slots sound different, and there is even a puzzle you can solve by hitting three crystal switches)! My only complaint is that turn at the end... not a fan of it.
I found the DLC courses to be pretty fun and I like the new additions to the karts. I don't think I'll be going back to that Sports Coupe now that I have the Blue Falcon... (though the Sports Coupe is faster)
I can't wait for the next set of DLC! I am hoping more Nintendo series will get representation in the next pack. Who knows? We might get Gaur Plain from Xenoblade as a track! (I hope!)
EDIT: I also noticed a lot of Links online and a lot of people were racing in the B Dasher, Blue Falcon, and Master Cycle.