I also picked up Boktai today. It's a neat game. It uses a solar sensor to detect how much sunlight is hitting the gamepak and your gun uses sunlight to power it. And it has to be sunlight... flourescents or incandescents won't work. So it was really easy to play at first this afternoon while I was sitting in direct sunlight... I tried fighting the boss when I got home and it was impossible due to the oncoming hurricane and ensuing cloudiness.

Looks like I won't be playing it tomorrow.. Oh well.