I just did a web search for my own edification, and thought I'd share what I found. In general, there are innumerable statements made by consumers saying things like "it couldn't hurt" and "it seems like a good idea." Then there are the claims by the manufacturers/sellers: these range from misleading to ludicrous. In my humble opinion, in an engine with an oil filter, this gadget is not likely to pay for itself over the life of the vehicle, nor is it likely to improve engine life or performance. If there was any proof they worked, someone, somewhere would show it. I cannot think of a single vehicle or engine renowned for its durability and reliability that owed its long life to an oil magnet.
This is a gimmick, on par with those ridiculous magnets people wear on their wrists.
That being said, I will be the first in line if anyone can provide proof that I should buy one.