I just have to reiterate: NO.
As a game that is billed as cooperatively multiplayer, this is one of the very worst-designed co-op games I've ever played. Characters occupy the same space and most levels contain surfaces or tunnels that only allow for a single character to stand on or squeeze through. Even if you are trying to help each other out, on any given level you are more likely to accidentally push a friendly character off a ledge to their death, steal their power-ups, or otherwise screw them up. I actually got a chance to play with 4 people over Christmas, and as I suspected, the result was even more hideous. Any time one character dies, as in single-player mode, the action pauses for half a second or so. If you are about to execute a jump that needs good timing (and virtually every jump in this game requires good timing), that little time jag will throw you off and, you guessed it, cause you to die.
Another fun-killer for co-op is the fact that the characters all look amazingly alike, so it is extremely easy to accidentally follow the wrong character for a second or two, thinking you are controlling them, while you are actually running your character into a pit of lava. Fun!
If inadvertent death and frustration are what you look for in a co-op game, then the new Super Mario Bros. is your cup of tea. Again, single-player mode or competitive mode most likely work much better, but some of us actually want to play alongside our friends.
If you want to play a game that actually implements co-op multiplayer in a good way, this year that game is Trine. It's a beautiful side-scrolling puzzle game with wonderful attention to detail. The art design is amazing, but the gameplay and puzzles are great too. In single-player mode you can toggle between three characters: a wizard, a thief, and a knight, each with their own strengths and weaknesses. The wizard isn't just a magic version of an archer, he can create metal boxes and platforms and move physical objects. To complete any given level, you have to use all three characters' at various times.
And multiplayer is great. If you're playing with two people, the two of you can be any combination of the two characters (but not the same character). So if your friend is the wizard, you can toggle between the thief and the knight. If you are currently the thief, they can transform into either the wizard or the knight.
Characters have distinct looks, so they are not easily confused. And they do not occupy the same physical space, so they can stand on the same narrow platform without knocking each other off. Gameplay is designed so that the adding players enhances the experience. One of you can cover the other from attacking skeletons while the other makes a bridge to get across a chasm. You actually feel like you are accomplishing goals together, not getting in each other's way. This is the fundamental principle that the game designers on the new Super Mario Bros. forgot (or just ignored).
The only real drawback to Trine is the controller setup. It probably works great on a Playstation, but on the PC you need XBox compatible controllers for additional players. A networked version would have been great, too. But purely in terms of game design, Trine is a perfect example of how you do co-op multiplayer right. And the new Super Mario Bros. is exactly how to screw it up.