Okay, I was "sold" on this product by an associate at a home improvement box store.  either I misunderstood, or he was uninformed.  I thought that the key included in the box was a "master" key, and that it could make another key(s) also work in the lock, but it turns out that once you use the original key to make another one work, the original one no longer works - making the new key the only working key.

So, an unintended consequence of this is an unscrupulous tenant could take another key of his/her liking, and "in 30 seconds" (as the vendor marketing claims) render his landlord's key useless by making his own key the "master key."  The rekey kits (with the little tool which I am sure can be improvised) are $10 in the box stores.

Anyone else see this as an issue, or had an experience similar to what I am describing?

Sure, tenants could change the locksets on any of us, but this takes waaaaaaay less time and effort to lock out the landlord.