Mario Kart DS is fun at times. They include a bunch of new courses, which some, sadly, replace fun courses from the N64 version. They have a 4 cups with 4 new tracks each, but some based on similar themes. I think I might have been a bit happier with an N64 remake.
Then you have a 4 Retro cups with 4 tracks each from the SNES days, GBA, N64, and Gamecube. I'm not to fond of these GBA or SNES tracks as they are short and simplistic. Some are intersting though, but there is no 3d to them. The N64 pics aren't the best ones. It's still an enjoyable game. Especially trying to get the unlockables. Because then you can have some real fun over the net.
They include missions though, but I think those are stupid. They exist to give the game a little more length, but they just seem pointless and they aren't all that fun or exciting.
Overall the game is fun, but my main dislike that really pisses me off is the 150cc engine class a.k.a. difficulty. 50cc is super easy, 100cc is a decent challenge, but 150cc... is just cheating. A bunch of cheating AI. They can speed up and slow down whenever they want to. They can crap out multiple items without multiple checkpoints or even picking up one of the boxes, which they often don't do. The AI has a habit of just ruining your last moments, before the race ends. And sometimes certain coincidences stop appearing as so. and instead I feel the game has a conspiracy against me and the AI is F-ing with my head. Just playing with me saying: I'll let you pass me through the final round and stay low behind you and then right before you cross the finish line I'll magically speed up and beat you too it.
Basically, I'm saying that this type of AI is BS and I'm amazed that they allow this to continue today. I've seen it in the N64 version and again in this version, it's a shame. For mulitplayer though, it's great. It's a fun game, but don't kill yourself trying to get the highest ratings or fully complete the game. It's a waste of time against a cheat.