Played up till halfway through World 2 tonight (single-player). So far it's, well, it's 2D Mario! It plays exactly how you think and know it will. It's fun, and already had a couple of quite gleeful levels so far, but I really look forward to some multiplayer with friends tomorrow.
My only gripe so far is that the motion controls can be a bit intrusive and tacked on; they kinda muck with the old-school vibe. To pick up an ice block, for instance, you hold 1 and shake the Wiimote; to throw it you let go of 1. So, to recap, you pick something up by shaking and pressing a button, but to throw, you just use a button, when a shake motion for a throw would seem natural. I realize that doesn't exactly sound harrowing, but it doesn't feel quite as intuitive as it should and I find my hands/brain occasionally getting a little confused. Also, I really like the NSMB main theme, so World 1 was a joy to listen to, but the main music in World 2...ugh, made me want to beat levels faster just so I'd no longer have to listen to it.
I believe them about ramped-up difficulty. I didn't die once in World 1, but there were a couple reasonably precise jumps for so early in the game. And I've already died a few times in World 2, although half the time it was from taking unnecessary risks for a big coin.
Anyway, so far it's a fun throwback, though not a revelation or anything--then again, it's not meant to be, except for the multiplayer, which I haven't tried yet. But still, there's a certain, uncontainable thrill in seeing--for instance--the Hammer Bros! In 3D!