Bowser Jr. is POINTLESS though. The Koopalings could have filled his role SO much better. They've got much more interesting personality, and it would have been nice to not fight the same boss OVER and OVER again. Nothing will make me not hate Bowser Jr. (unless he finally learns to share the spotlight with the Koopalings).
Bowser Junior is a thrice redundant character. He is redundant as a miniature version of Bowser, because we already had Baby Bowser. He is redundant as a child of Bowser, because we already had the Koopalings. His alter ego Shadow Mario is redundant as an evil version of Mario, because we already had Wario. That's why I hate the character Bowser Junior. Nintendo seems to think we like him, though, since they keep putting him in Mario games. Rumor is he's going to be a playable character in Brawl... ug...
Anywise, I just finsihed NSMB (I think) and it was good but not great. There were only a handful of levels that really impressed me. The game was much too easy. I never got a game over once, and I have close to 70 lives on my file. Lives were basically meaningless in this game. When lives are meaningless, 1-ups are meaningless. When 1-ups are meaningless, coins are meaningless. When coins are meaningless, 90% of the game is meaningless. I still tried to collect coins, of course, but only for the sake of collecting them. I miss the days of Super Mario Brothers 3 when coins and 1-ups actually mattered. The bosses were laughable, and Bowser Junior made the game orders of magnitude less enjoyable by his very presence.
My other main complaint, besides the difficulty, was that the new elements in the game seemed under-utilized. The blue shell, for example, was a very cool idea, but I had very few reasons to actually use it. One of the first things I noticed when I used the miniature mushroom was that I could bounce off enemies without killing them. The game never really made any use of that mechanic. The miniature mushroom was used a lot, but only to make Mario small (and in a few cases, to allow him to run across water). They could have done a lot more with the miniature mushroom and the blue shell.
I have to give credit where it is due, though. I loved the music, and to my own surprise I loved the graphics. A few levels really did impress me (2-6, for example).
It was a fine game. I don't regret buying it. But, it pales in comparison to Super Mario Brothers 3.