We tried those when I worked at a library because we often had patrons' info up on the screen, but paradoxically they were no good for helping patrons (because the patrons needed to see the screen along with us). I also played with one on a friend's laptop, but the viewing angle IMO was so narrow that I felt very restricted.
My friend needed it because he works with confidential info and has his laptop out as he commutes via train, so I can understand the use when confidentiality makes it necessary. He was happy enough with it, so it served his purpose. Personally I found it very irritating.
