Yes megapixels are useful as the more you have the higher the resolution of the picture. This means you can zoom in after you've taken it without losing as much quality. It also means you can blow the picture up large and still have a decent result – making it great if you plan to print and frame your snaps.
But the megapixel count isn't everything, so keep that in mind. The ability to handle low light, measured by that "f/" aperture number is also very important.