mario vs. donkey kong minis march again! is very good, and it is only 8$
Yes Mario vs Donkey Kong is worth the $8, as it provides many hours of puzzle action. It even lets you create and share levels, and download other people's levels.
The only other game I've purchased is Dr. Mario Express, and while it's a pretty basic version of the game (it only has single player classic and vs. cpu), Dr. Mario is one of my favorite puzzle games of all time and still provides plenty of enjoyment for 500 pts.
I don't know if it really counts, but I liked the brain age: math enough to buy the entire brain age 2 game. I know they catch alot of hell, but I kinda dig the mario clock which was only 2 bucks.
Music by YMCK
Homage to NES games
Mix of Picross and Tetris
Totally worth the $5