Its a great way to lower the front and make it look equal to the back. Basically it just makes the spring sit a little lower on the shock so theres no real danger there. They are pretty safe.
Common applications would be with a set a springs, let say and you still find that the front has a little more gap than the back, then this is the solution.
A couple of people had them on their Audis on Audizine. Very popular modification in Europe too.