I doubt I'd be a very "fun" owner if one was dropped to earth. Simply put, for all of my ranting about militarism, and using violence to solve problems, I tend to make such arguementson a societal level. I don't think that it's a good thing for one person to play god.
I haven't seen the entire Anime series (but I've seen a bit of it) so I can't answer a lot of the detailed questions/scenarios other people have come up with, but the bottom line is that I have no illusions about being a strong, good, or incredibly moral person. I don't think I'd trust myself to make use of it.
What's more, if your dealing with an item like this that is tied to demonic forces, that would be a physical, demonstratable proof of an afterlife, the wicked being punished, and so on. I'm in a dubious enough position as it is morality wise, without mystically killing people. Even if I somehow took over the world and gained everything, when it's time for me to go where am I liable to wind up? Whether it's a western hell, or a Japanese one, eternal torment is eternal torment, and there really isn't anything worth that. I suppose the most worthwhile aspect of being offered a Death Note, would be to find redemption through rejecting it/refusing to use it.
It's sort of like another Anime series called "Hell Girl" (of which I've only seen a few episodes). Sure, you can summon the demon to kill anyone, but you wind up just as badly off as the person you send it after, so what's the point? But then again I kind of figured that was the whole idea behind that series.