Mario 64 DS feels like a fan hack, and I mean that in the best way possible.
It's so neat to see a game I like recontextualized in such a novel way. It's got new levels, some really cool new bosses, and it lets you play as three entirely new characters.
The only real issue I have with it is the fact that it's still Mario 64, so everything I said in my review of the original still applies.
Still, a valiant effort.