I prefer the N64 version. Sure, the new one has features and graphics, but it's just not as fun without the joystick and the big screen. And I liked having only Mario playable instead of having to switch between the characters for their abilities.
I don't like the DS version. I love the N64 version. If they took the N64 version and directly (without any changes other than the graphics) put it onto a DS version then I would have bought it.
I don't like the new changes, Luigi, Yoshi and Wario just throw off the game. I'll stick with my N64 version.
Galaxy is very very good. A true modern classic.
I love Mario64, and it was a nice surprise to start out as yoshi, but it was a bit annoying- as much as I love yoshi, I do prefer being mario in mario64. Top quality game though, and it's great to hear that catchy music from the first levels again!
IMHO Galaxy is too gimicky at times, and the levels are much less open than SM64's. Still an excellent game though.