Super Mario Land 3D is the best Mario game I've played in years. Everyone knows Mario, so I'm not even gonna bother summarizing the story or anything. Let's start with the graphics. They're good. Very good for a 3DS game. Next subject. Sound/music. It was also very good. With many remixes of familiar songs and a few new memorable themes, you'll be whistling like a whimsical little kid while playing the game, and even sometimes afterwards.
Gameplay. Obviously the most important part. The gameplay was the best in a Mario game since Super Mario Sunshine in my opinion. Sure Galaxy was fun and all, but the gravity puzzles got annoying and it just didn't have the same feel that earlier titles in the series had. SML3D goes back to the more classic gameplay style bringing back the basic run and jump controls from the 2D games into a 3D plain. The 3D effect, while admittedly the best use of the feature to date, still doesn't really add anything special to the game and most gamers will probably choose to play without it for the majortiy of their playthrough.
Replay Value. This is the part that really impressed me. At first I was a little saddened by the seemingly short length of the game. But once you beat the game, you unlock with is essentially Super Mario Land 3D: Master Quest, adding a whole new level of awesome to the game. Most, if not all of the levels are fun to play over again, unlike many of the stages from Mario Galaxy or New Super Mario Bros, and this is only helped by the three hidden golden coins in each level and the option to play as Luigi after your first playthough.
So yeah. Super Mario Land 3D. Great game. A must-have for all 3DS owners. If you have one, go out and get it. If you don't, well then that's too bad. Play a friend's copy some time. If you already have it, then good for you. Super Mario Land 3D gets a 9/10.