So I'm probably too old to play Mario. That doesn't stop me, I can discuss baddies with my six year old.
Having owned a 3DS and Super Marioland 3D for one day, I feel qualified to comment on it. I may possibly have spent rather a lot of that day playing it, of course.
The 3D aspect is a gimmick, quite cool but doesn't really add or detract anything to gameplay.
This is a standard Mario game with progression through worlds and levels, with a boss level at the end. The appearance is a lot like Mario Galaxy but playing is more Super Mario DS. One DS detail is that Mario can stow an additional power up item, but in this game he can switch back and forth between the item he is using and the one in the box.
So far Mario has mostly used his raccoon power up as he has a lot of levels that floating helps on. This also allows him to kill enemies with a swish of his tail. After a few failed attempts at a level, a box appears with a gold raccoon leaf in it, this makes Mario invincible and with all the raccoon powers. It does not make him invincible to falling off edges and I do that a lot, leading to the discovery of a goal flag, which takes you straight to the goal. I haven't tried that, seems a bit pointless.
As usual, Nintendo have included a homage to past Mario titles; one level has the classic stepped brick climb to the flag at the end, except here it is 3D and you can walk around it. Which is nifty.
The gameplay here is good, it is very much a 3D version of the old DS gam and fits well as the next title in the series.
Good points:
It's a Mario game.
Ability to save and swap power ups is a great feature.
Cheaty invincible raccoon thing means you can finish a level you're stuck on, but still collect medals and actually do the level.
It saves after every level.
The controls are pretty instinctive, except power jump.
Bad things:
I've finished world 2 after only a day. The levels are too short and not that hard.
The power jump is located on the left and right paddles. As these are easy to press by accident, Mario often plummets to his death halfway through a jump. Very, very annoying.
As to the 3DS, it's everything you'd expect. The 3D is quite cool, being able to take 3D photos is pretty exciting, although I haven't tried to take them off the 3DS yet. The included games are a bit rubbish, and I'm not sure I'm comfortable with Streetpass automatically sharing stuff with anyone that also has a 3DS. I'm guessing that the average demographic of 3DS users differs somewhat from me, but I live in hope to receive a new Mii.
Yes, Mii. I love Miis. This can be created from a photo on the 3DS, so it's actually quite good.
I don't know how this compares to the DSi as I've never owned one, but it compares very well to the DS Lite. And of course if you want to play the new Mario titles, including Mariokart 7, which will be purchased very soon, then you need it. Yes, need.