As a creation tool, it's absolutely fantastic! While the level creator itself is not nearly as complex and intricate as, say,
LittleBigPlanet 2's is, and won't lead to creations that are as complex and creative as that game's best user-created levels, to say that it's much more user-friendly is a huge understatement. The whole drag-and-drop interface, made possible on consoles thanks to the Wii U's gamepad, really makes a difference here and allows you to make levels in almost no time at all (whether or not they're any good depends on your skill and creativity, obviously). A near-5/5-worthy creation tool if I've ever seen one, even if it still lacks slopes at the time of this writing.
As for the mechanics in the levels... well, it's Mario. Four different styles. While the mechanics of
Super Mario World [スーパーマリオワールド] aren't perfectly emulated here (you can't spin jump on buzzsaws, for example), it's more than acceptable, and the mechanics of
Super Mario Bros. [スーパーマリオブラザーズ],
Super Mario Bros. 3 [スーパーマリオブラザーズ3], and
New Super Mario Bros. Wii transfer over perfectly fine (to my knowledge. I have a feeling that those who are obsessed with them could write a dozen essays about how this is terrible in transferring them over). And thankfully, in the case of the SMB1 and NSMB styles, due to the options available and the fact that this is a user-creation tool, you can easily make levels that are much more interesting than the ones available in the original games (at least in my own opinion. I was personally never the biggest fan of those games to begin with, though)!
Playing the levels of others isn't consistently the best of experiences, though. At all. This is to absolutely be expected due to half of this being a user-creation tool, with said expectation being larger due to this being a Mario game and is thus inherently marketed towards children, but the majority of the levels that are present at the moment... really suck. If you're just going around internet communities that share their levels, this isn't too big of a problem, but if you want to unlock most of those neat Amiibo outfits to use in SMB1-style levels, you're essentially going to have to jump into a pit of lava. The 100 Mario Challenge that you'll need to play through multiple times (at least 50+ times, even if you're an Amiibo hoarder) has various difficulties, each with their own problems: Easy will often give you levels that are either obviously a young (like...
really young) person's first attempt at making a level in any game period or one of the many currently-existing automatic levels, that are cool the first couple of times you see one but quickly become a bore. Normal is a somehow bigger mixed bag as it's a mixture of levels fresh off of the creator and ones with a decent clear rate. Anything goes here, though this is probably your best chance at finding something good via the Challenge. Expert is typically filled with Kaizo-esque bullshit, only not nearly as creative as the levels in the Kaizo ROMhacks themselves. Thankfully, good levels do exist, and randomly stumbling upon them feels really good and makes it almost worth it. Almost. Well, kind of, since you're usually catapulted back into a shit level afterwards unless it's the end of a 100 Mario Challenge, but that's enough about that.
I'm personally not the biggest Mario fan out there. I mean, I like the series, but I personally wouldn't call it one of the greatest things to grace the medium. But this has a
lot of longevity in it. Even more so due to the fact that Wii U emulators are becoming a thing, and the way that this saves levels means that people could very well be sharing levels even long after Nintendo eventually shuts down the official servers. Even if my enjoyment during sessions of playing levels fluctuates constantly due to the variety of levels quality-wise, it's still worth it in my opinion (well, maybe not for
$60 unless you're a huge Mario fanboy or are really interested in a fantastic creation tool. Maybe wait for the inevitable price drop otherwise). Unless you don't like Mario, obviously.
tl;dr: As a level creation tool, it's kind of incredible. As a Mario game, mechanically it's fine (but not amazing. Though I personally don't think that any of the original games this uses the mechanics and style of were 5/5 material in general, even if SMW comes kind of close) and level-wise it's obviously a mixed bag.
THAT FUCKIN BIRD THAT I HATE
THAT FUCKIN BIRD THAT I HATE