I've been stuck playing PSP and DS agmes for months, but this game is the first time since November of last year that I've been glued to a GBA game. It's fun, innovative, good for traveling (with an easily accessed sleep mode at any time) and cheap. $14.99 can't be beat.
Basically, it's like a shortened Yoshi's Island with lots of small stages (again good for portable play) which uses the tilt function in lots of creative ways (tilt walls so you can walk on them, tilt to unroll bridges, tilt cannons to direct which way you're going). Yoshi can turn into several different forms that also take advatage of the tilt function (like a balloon that floats in the direction of the tilt).
Good fun, I recommend it for the price.