Window films enhance privacy by reducing the transmission of visible light (making the glass darker) or by increasing the surface reflectivity (making the glass more mirror-like).  If you are trying to create privacy, you have several choices and limitations to consider.

Exterior Windows:

Window films may provide privacy during the daytime, when it is substantially brighter outside than it is on the inside.  At night, this effect reverses (see example below).  When interior lights are turned on at night, persons on the outside will be able to see right in.  In addition, it may be difficult for persons on the inside to see out at night if the interior lighting is bright!  This is due to the laws of physics and no window film can overcome this.  If you need privacy at night, consider a Venetian blind or a curtain.
 

During The Daytime
If you are outside, it is hard to see inside
If you are inside, it is easy to see outside


 

After Sunset - Night
If you are outside, it is easy to see inside
If you are inside, it may be hard to see outside


 

Now consider a typical interior situation where a one-way mirror effect is desired.  There are two important elements which must be in place.  First, we need to increase the surface reflectivity of the glass.  This is easily accomplished by the application of Silver 15% film.  Next (and just as important) you need to control the light ratio.   Consider this conference room example:

How much darker does it have to be on our side?  At least 8 times darker in order for the effect to work well.

This is also due to the laws of physics which can not be overcome by any window film.