Well the good things about the DS:
~Portable, you can take it wherever you want and play it whenever, you can play it when you wake up, when you're in bed, when you're at school/work lol.
~Pokemon is the greatest RPG battling game ever made.
~Wi-Fiable. I actually play at school.
~Connects wirelessly to other DS's and also the Wii.
~Can play multiplayer with 1 game in lots of cases.
~Many tournaments for stuff like Metroid Prime Hunters, Mario Kart: DS, Tetris DS and soon to be Pokemon DP.
~Well graphics obviously aren't as good as any console / PC.
~Harder to control for some people, especially stuff like aim in FPSs.
~Not as easy to Multiplayer than say a console; you need friends with DSs unless you have Wi-Fi.
All I can think of now. Obviously me being a DS owner (esp. getting one for free) would recommend getting it.
Well I think you mean New Super Mario Bros not the N64 game Super Mario 64. That game has had real good ratings and the highest selling DS game before Pokemon DP. If you like long single player games I guess it'll be really good, I haven't played so I don't have much to comment sorry.
Diddy Kong Racing: Well the fact that everyone plays MKDS there are a lot less people who play this. I don't know if this is from advertising, reputation or just plain gameplay. Can't really comment here. I know that on N64 it and MK64 are on pretty much equal footing. The cool think with Diddy Kong Racing on N64 is the choice of vehicle, not too sure if it's like that on the DS.
And finally if you do buy a DS, GET POKEMON! It is a must.