Ridiculously difficult... but that's why it's so good.
It sucks you in the very moment you start playing, and no matter how tough it gets, you just can't stop playing save for the odd occasion where you need to sleep or eat. You'll need to play the first few levels at least 10 times each before you reach the goal, but this gives you a huge sense of satisfaction.
Every fourth level is a "fortress level" that involves hours of exploration until you find your way to the boss. The bosses were somewhat disappointing, but still good. After the fourth stage, the seemingly unrelenting difficulty finally lets up. The fortress levels are still damn near impossible, though.
The satisfaction of finally beating the last boss makes this game well worth the approximate $5 US you paid for it. If you can't stand difficult games, stay away from this game. But if you're the kind of gamer who loves a good challenge, Kid Icarus is a must-have. I would give this game a 9.5 (which goes against the automatic score given based on what I gave each category, since gameplay is far more important than graphics).