I bought a set of these for a used rack I picked up on craigslist. Too late did I realize that they don't work on Sportworks racks, which were the previous generation of the Thule T2. I have a hybrid rack - 1 Sportworks carrier and 1 Thule T2 carrier, so I will only be using one of these locking cores.
So I installed the locking core, and put my bike on the rack to try it out. As other online reviews posted (but I ignored) all you really have to do is remove the front wheel off the bike to totally bypass the locking mechanism. This really is true...all you have to do to unlock the bike from the rack is to remove the front wheel! This core is a deterrent, but is not nearly as good a safety measure as something like a U-lock.
My recommendation? Skip these cores all-together and spend your money on regular chain or U-locks to keep your bikes safe.