These deflectors are useful if these are of proper design. There are certain designs available, specially for Mehran and other small cars, which are too bold and big. They normally don't serve the purpose of deflecting air, just 'Show Sha'!
However, flater and smoother ones are good for deflecting air while window is almost half open. Very less air comes in. And other benefits are already explained above.
One problem I have noticed is that these are really fragile! I mean within 2 to 3 months, these deflectors start cracking and breaking, not by itself, but if someone including myself is likely to put a hand on one, pushing it inward and cracking it!
I changed two sets in past six months!