It's pretty stellar, the NES games are perfect from what I've seen, with the only exception is how easy it is to accidentally end the game. Sometimes while the magic/item subscreens are displayed I accidentally hit X, and those endgame menus appear (the ones that appeared when you hit select or down and a or something on player 2) and you can't continue your game, these endgames count as a death on your death count. Through 6 palaces, I accidentally ended the game 3 times in The Adventure of Link.
Ocarina of Time is pretty much perfect, but the control seems to sensitive from my playing with my worn out N64 analog stick, but that's subjective i suppose. I haven't really gotten into Majoras Mask, but there is a warning that tells you that the game isn't perfect, and that slowdown will occur, that being normal. I hear that the game suffers from some of the worst slowdown ever seen on a GC.
The retrospective is nice, makes you want to play some of the titles you haven't played (gb oracle games, for me), but it isn't really spectacular.
And then there's the Windwaker demo. Played a little bit, not really interested.
Btw, you can choose these games at the start through a a horizontally scrolling menu.
For my favorite zelda, adventure of link, and the main draw for me, it's stellar.