I find the DLC to be a mixed bag. I don't care for any of the "characters" it brings, but some of the vehicles are nice. They should all have come with tires and a glider though, but I have that complaint about the base selection as well. As for the tracks...
-Yoshi Circuit: They probably should have spiced this one up more, but even still, this was one of my faves from Double Dash!! so it's still fun.
-ExciteBike Arena: I like the concept, but feel the track should be longer. This just makes me want a new Excite game though.
-Dragon Driftway: This one has some fun twists and turns, but I don't like the theme of it. The whole Chinese motif for the graphics and music just doesn't fit at all, I don't get it.
-Mute City: This one definitely captures the soulless feel of an F-Zero game, heh heh. Aside from the boring visuals it's fun to race on, but I felt like item spamming was bad here. Or maybe it was just an unlucky period.
-Wario's Gold Mine: I don't really like the changes they made to this one, it feels less dynamic and easier compared to the original. The original was fun because it took some skill and had some danger.
-SNES Rainbow Road: This one feels like a throwaway, it's a real stinker.
-Ice Ice Outpost: I like the idea of the two "lanes" thing throughout this track, but it seems a tad plain. There should have been an underwater section here at least. There are a couple shortcuts that I haven't figured out how to take properly yet, but, they seem pretty dangerous for the small payoff.
-Hyrule Circuit: Probably the best of the bunch, this one just has a nice flow to it and is fun to race, not to mention the nice Zelda nods.
Overall, the DLC is a bit underwhelming and the tracks don't seem up to the quality of most of the new tracks in the base game, but still worthwhile.