I'm going to elaborate on some of the things jakubr said.
1. Well you can perform it at day it will not look as good, not because it won't work as well, just because its less "creepy" if you know what I mean.
2. It works on almost any phone. I cannot think of any phone it won't work on. You can also take a video of the "ghost" moving around.
3. There are no clothing requirements.
4. It can't be performed completely surrounded, just because there needs to be a clear space for the "ghost" to be. Even if they see the "thing" it is still pretty hard to tell that its something sneaky, unless you look real close.
5. It is easy, but will some figuring out of the phone you are using.
It can also be performed as a reveal of a chosen card or thought of word.
And yes, I think it's worth buying.