in my VW 1.8T, I use them and they do a good job at catching iron particles. I take mine out after each oil change and wipe the particles onto a paper towel. I use this to watch for how much particulate there is. They are reusable for at least several OCIs before their magnetic strength decreases significantly.
I also use a FilterMag which catches even more particles but checking how much is caught obviously requires cutting the filter open. I did that once to verify it worked.
I also use a drain plug magnet. It too catches particles even though the other two magnets are also in use. It's easy to wipe those particles onto a paper towel so is worth it to me even though this magnet catches the least amount of particles of the three.