FWIW, my review of the game (assuming the last 2 worlds don't completely blow my mind):
My brother and I played for about 5 hours overnight and 2 hours the next day (we tried to be thorough as we went through every stage, and we completed world 6). Here's my stab at a brief review.
1. What the game does right:
Multiplayer in a 3D Mario game is a huge first, and in our case it was a lot of fun (both of us are experienced gamers, so playing through levels with someone of equal skill was really fun). The areas that were the most enjoyable in multiplayer were the ones where we had to coordinate with each other to progress (or to get secrets). For example, the Plessie levels where we had to control the dino together, or the Red / Blue jump flip levels. Of course, there are more minor moments of this throughout, like when one person has the cat suit, but the the other person notices a green star that you need the cat suit to get to. Communication is needed and it's fun in a 3D platformer.
The variety in the game is excellent. The amount of time between re-used ideas was significant, and when they were reused, it was in a different / more challenging way. For example, the first time you ride Plessie, versus the second.
Level length is perfect. The size of each level feels like a perfect bite - not too long, not too short. More like Super Mario 3, rather than Mario World.
Multiplayer has greater depth due to the different skills of each character.
The new way of navigating the overworld map is a small, but nice improvement, since you can explore and find little secrets here and there.
The cat suit is a fun powerup that invites you to explore even further.
2. What the game doesn't do so well
The characters move too slowly, which has several impacts. The way it's implemented now, the "full speed" sprint is the only speed that feels good, and since you have to work your way up to it, it always feels like you're fighting against some sort of molasses to get there. Second, sometimes the "full speed" burst kicks when I didn't realize it would, and it messes up the platforming.
Using the Wiimote Dpad to control characters in a 3D space sucks, and the fact that the game is often in an isometric viewpoint (which looks great) means that you'll often be pressing to the right and up on the Dpad at the same time - not the most comfortable position. Also, it sometimes felt like I was pushing to the right, but I was really also triggering the up direction, which made the side scroll sections, especially to the flagpole, often a "miss" for me as I would go off in the vertical direction slight and mess up my aim. My brother, who played with the Gamepad, had no such problems, and when I tried playing with the Gamepad, it was much better. Not sure if the Pro controller requires you to use the Dpad -- if it doesn't (meaning, if it lets you use the analog stick), I would go as far as to say that it's the only real way to play the game (Gamepad + Pro Controllers).
These two things (especially the Wiimote DPad issue) broke the experience for me, I'm sad to say.
Final Verdict: Playing with the Gamepad: 9/10; Playing with Wiimotes: 8/10.