It's a wonderfully constructed game. It's very challenging, but never frustratingly so. Out of all games I've played, nearly none have nailed difficulty better than Super Luigi U.
For one thing, the length of the maps is really golden. A large problem with the difficult maps in previous 2D Marios (and nearly all 2D platformers in general) has been that in order to get to the difficult bits, you have to run through 30-60 seconds of relatively mundane stuff. In Super Luigi U, they just removed the mundane stuff, leaving only the great stuff.
Secondly, and possibly more importantly, how you adress each map nearly always changes, and you could always have done it differently than you did. You start out with one strategy, struggle a bit, before you find and try a new approach. Maybe it's better, maybe it isn't. But you're never stuck just banging your head against a brick wall trying to do the exact same thing over and over, because you can always just adapt a different strategy and try that.
Being able to find and choose your own path - rather than there just being one single path - has always been Mario platformer's arguably biggest strength, but it's even more apparent in Super Luigi U.
It's a glorious, difficult feelgood game, and the perfect way to teach people platformers. New Super Mario U + Super Luigi U together feature a fantastic learning curve. Starting out in World 1 of NSMB U easy enough for anyone to play it, gradually becoming more challenging up to the final boss, and the bonus (World 9) missions being properly difficult. Then SLU continues the increase in difficulty, and by the time you finish it, you're properly good at platformers. If you made up your mind to create a game (or in this case, two) with the goal of teaching people to play platformers, you couldn't do it better than NSMB+SLU does.
