Picked up the game today, and have so far completed World 1 with all the optional Star Coins.
After getting over the initial shock that this wasn't running in 60fps (the Galaxy titles spoiled me), I've learned to like the look of the game. At the cost of a lower framerate, there's better shading effects. (The Thwomps have never looked better!) The 3D is very tastefully done, not at all intrusive (this might be the first 3DS game where I have the slider set to max), and yes, it really does help in gauging certain jumps more accurately. There's a real thrill in leaping from dozens of feet in the air, and hitting your enemy's head from way up high -- almost a Jumping Flash kind of airtime rush. :) Framerate remains the same whether you have the 3D on or not, so I have no qualms with just leaving it on.
It's refreshing to play a new Mario game that doesn't bludgeon you with countless tutorials. In fact, there's practically none at all: you're pretty much left to your own devices on how to do everything. Luckily, if you're any kind of Super Mario vet, you'll be able to figure it all out in a heartbeat. (But here's one tip that may save you from a preemptive death: the Tanooki suit now floats by HOLDING DOWN the jump button, not mashing on it a la SMB3.)
Challenge wise, it's been an easy affair, but the level designs are incredibly enjoyable. The direct paths to the goals are linear, but the game's chock full of deviating paths that are always guaranteed to reward you for your curious diligence. Be it a Star Coin, a power-up, or a 1up, there's nooks and crannies EVERYWHERE -- and I found myself playing each level in the first world at least two or three times just to find them all.
Unlike the two New Super Mario Bros. titles, the game thankfully has a Save Anywhere feature. It also auto saves after every level.
Flaws? You'll still amass an absurd number of extra lives in a short period of time. At last count, I had 42 lives from the start of World 2. Also, the new castle theme is disappointingly lackluster, especially when compared to the awesome ones featured in the two NSMBs.