I'm sure I'll be the bad example here but I don't use one. I have an attached garage and when we moved here 20+ years ago, I built one on wheels. It was essentially a mobile welding bench with a fume hood over it made from two range fans (I'm guessing about 150 CFM each). About the same time I switched from O/A and stick to MIG and TIG—and there just weren't enough fumes to worry about it. As pointed out it needs to be right over the weld area to be effective, but not too powerful that it sucks the shielding gas away. Even when I had my plasma table I didn't need it. I guess it's because for the type of hobby welding I do, and for the length of time I am striking an arc, I don't generate enough fumes to lose sleep over. If I am welding for longer periods of time I just open the garage door. If I were doing so much welding that I needed one, I'd be crossing the line from hobby to commercial venture, which voids your homeowner's insurance in almost all cases.
I'm sure there is some detailed medical study out there that documents how even a small amount of fume causes cancer. But at this point in my life, reading these studies is more harmful to me than the fumes I inhale. As I said, I'm sure I'm a poor example, but at the price of these extractors and for a hobbyist, they are out of my league.
Just my 2 cents....if I were younger and smarter I'd probably feel differently, or avoid the hobby altogether. For people who have made their own, good for you...maybe it extracts paint fumes as well which would be a good thing.