Meh I wasn't that impressed.
For more on that here's my "first impressions" input from the product registration survey:
While always fun to play a new Mario adventure... This one just didn't stand out. I just sat down and beat the entire main game in one short sitting. And when I say beat I don't mean I rushed through to get to the end, I mean I took my time and one hundred percented every single level on the way. The two words that instantly pop to mind are "uninspired" and "obligatory". There was nothing compelling about the games presentation or gameplay - the whole game just kind of felt unnecessary. "We have a new system we better have a new Mario to go with it - lets just quickly squeeze one out so we can move on to bigger and better things" must have been said more than a few times during development. Though the music was beautiful as always and I have to admit the special world levels were acceptable. While being nothing more than recycled material slightly, slightly modified it still kicked you while you were down and didn't ask for forgiveness. But I also believe that Luigi being playable should have been more than an aesthetic thing - In many of the last games Luigi distinctly played differently in a more challenging way which added to the fun. Being nothing more than a Mario clone is old and presumed dead - why move backwards?
I certainly had fun playing it though. The actual gameplay kinda felt like a cross between mario 3 and mario world but with depth added.