I know I'm a little late to the party, but I just picked up SMG a couple of days ago. I've only got 10 stars so far, but I thought I'd post a quick initial impression.
I haven't really enjoyed a Mario game since Super Mario World on the Super Nintendo (I'm really a 2-D kind of guy), and this includes Mario 64. The 3-D ones have been OK, but I'm one of the 2% of people who actually suffers from motion sickness from 3-D games.
But SMG is fun. The graphics are quite impressive for the Wii, and the controls are instantly familiar. I like that they've simplified a lot of the moves, as I can focus more on the gameplay than trying to mash buttons or push sticks at the exact time that I need to.
I find that the upside down and sideways portions still leave me feeling a little dizzy and disoriented. But as long as I get up and walk away every few minutes, I'm ready to come back for more.
My 6- and 4-year old boys are enjoying it as well (they love Mario 64), but they're finding it a little difficult so far. They keep handing me the controls and asking me to help. But they're catching on, and I think with more practice they'll be able to hold their own.
Overall, it's well worth the price of admission. It's been a while since I've found myself thinking about a game and counting down the minutes until I can go play again. I think Nintendo has finally found a game that's going to sell the console other than the included Wii Sports.