I've had 2 in the last 5 years, both from Harbor F. The first one was the 6 function $40 dollar model; used it to clean up roofing nails after having the house shingled, ended up giving it to a friend afterwards. The 2nd one was the 9 function $45 dollar model (moving up in the world); got that one a few months ago. Again for yard cleanup. This time it was stripping and replacing the vinyl siding that left tire popping debris everywhere. Everyone says "well why don't you use a magnet" which is/was a great idea until I found aluminum and stainless hardware.
I forgot how many nails I found for the roofing job, but I'm up to 7 lbs from the siding and I'm still not done. With 2 trailers, a lawn tractor, a compact tractor, a dune buggy, my pickup & the wife's jeep frequently traversing the yard I can only imagine how many headaches those detectors have prevented. Other than the nails / screws I found 1 penny, 2 hex nuts, a cotter pin.