Over the last several years, I and others have talked about the promise of unikernels â€” single-purpose, lightweight virtual machines â€” in the cloud. However, all of these talks have simply presented our architectures and speculated about their usefulness. Over the last several years, Galois has actually been using unikernels to implement interesting components in critical systems: non-bypassable encryption components, network monitors and alarms, platform obfuscation capabilities, Tor nodes, network re-routers, and so on. In this talk, I will speak briefly on each of them and ask the question: Was a unikernel a good platform for this project? If so, why? If not, why not? What are the general rules we can infer about when unikernels are useful, and what part of the cloud ecosystem they are best suited to serve?