And it's out on 3DS Virtual Console this week.
 
If you don't have it, get it! A really great addictive game, and it's also really long.

I'm going to jump in and agree with this wholeheartedly.

I had this game when I was a kid, and I knew it was well-regarded now, so I ended up using the extra four bucks I had after purchasing Link's Awakening on this. I expected it to be good, but I wasn't expecting it to outshine LA and have me drawing comparisons to Portal in my head (in terms of thinking about how to solve puzzles, this game obviously doesn't have the clever, snappy writing which serves as Portal's foundation.) Every level in the game is a contained puzzle with one goal - grab the key and carry it to the door which leads to the next level. You accomplish this using Mario's myriad variety of jumps and flips and the tools in the level provided to you. Later levels become particularly devilish, using combinations of timed platforms, ladders, blocks, jump springs, and switches to get the key to the door. If you enjoy the mix of contained logic puzzles and action Portal brings and you have 4 bucks to blow on your 3ds you should get this immediately.