This is quite a great device in theory. In practice, it's very well made and mostly does the job. My complaints are pretty minor; first, due to where the magnetic strip is located, it will not hold a small paring knife well. Second, you really have to slide the knife up and off, which might not work in certain setups (under a cabinet, for example). And third, if the base had just a bit more mass to it, it would be far more forgiving of slide vs. twist.
This is also a bit too small to hold a full knife set, but does perfectly with a couple of santokus and one or two chef's knives. The magnetic grip is very good, but realize the section at the top has about two inches of non-magnetic area which can cause some issues under very specific use-cases.
All in all, for the price, it was a good buy and I do recommend it. It is not, however, without flaws.