i have it and it's a very good game, only reason i dont play it now months later, is that it's gotten to the point where it's too hard for me to get any further.
The music and graphics are top notch, the levels are well thought-out, except multiplayer sucks.
I bought it for $80 australian, (so $US40 would be your equivalent) and i reckon it's worth that.
btw, being a games programmer, i cant stand the thought that people will blatantly rip off my product without supporting the industry, however i think the problem actually lies in the way that a lot of studios are making really bad games, slapping a license on it and calling it a day. I personally dont buy games for the ds anymore due to hundreds of dollars spent on poo. If you want my opinion, rent or burn first, then if you still want to play it 2 days later, buy it.