Are hand held AC power testers (the ones that looks like a screwdriver) completely safe? Why/why not?
These have been around since Adam were a lad, and yet you never hear "Another home owner died today using an AC test screwdriver!" on the news.

The truth is, like any tool, if you maintain them properly, and use them as intended, the chances of them hurting you is minor, but not zero.

The thing to remember with these though is the logic is... 

"If you see a light, it is hot. If you do not see a light, it might not be hot." 

That means, never rely on one to test a wire before you grab it with your fingers.