The game is pretty fun. I think the multiplayer is the best aspect of it.
The game has its flaws, though. Like NSMB for the DS, the most places lack atmosphere and the music is horrendous.
Also the secrets, while better than those on NSMB for the DS, aren't really that exciting.
The use of yoshi's is frivolous and unecessary, unlike Super Mario World. When you get a yoshi, you can't keep it after that level. Also, there aren't different types of Yoshi's.
The game isn't really that difficult. To those saying that it is difficult than SMB 1 or 3, I say no way. At one point I had almost 90 lives. I had the most difficulty on the last level of world 8, but that was probaby because I was playing it at 4 in the morning. Even so, it wasn't that hard.
Even with all that said, I still recommend the game, as it is good fun, especially when playing with others. Multiplayer takes a bit to get used to though, as it is easy to get in each others way (jump on each others head, take powerups, etc). Once you get the hang of it though, multiplayer adds some good advantages.