Without a doubt, RB3 is the best choice of music game available on the Wii to date.
A step up on Rock Band 2, the game boasts better graphics, a streamlined interface and continues to have an impressive song set – within the game and as extra paid for DLC. You can also import songs from Rock Band 2 and Green Day: Rock Band, which is a seamless process. While the Guitar Hero series seems to continue with “more of the same,” Harmonix has added further gameplay through the “Pro Mode.”
It's not without fault, there seems to be an odd slowdown once a song ends and I've played a couple of songs where there has been a hovering guitar and nothing where a band member should be. While it is a step up on the Wii to import songs from previous titles, you are currently restricted to 2 prior games. The other consoles allow you to import AC/DC Rock Band, Rock Band 1 and Lego Rock Band. Hopefully, they will fix this in the future. It also requires a significant investment in new hardware if you want to play the Pro mode (which I don't think is available yet.)
It's a step in the right direction and a definite must buy if you're invested in Wii music games.