I have tried and tested them and they do give big improvements... even more if they have a reflector attached to them. The picture that I am showing is just me testing the WIFi signal gains. I put the biquad antenna in a container because they are fragile but the gains I got surpass the antenna it came with. Adding a reflector further improves the signal...
The guy in the video is an expert at this stuff... go check out his other videos... 
I also like to remove my USB WiFi cards and take readings of their temperature if they are not over heating because improper heat dissipation causes signal issues too... 