VMware has published a technical white paper about vSphere 6 Fault Tolerance architecture and performance. The paper describes which types of applications work best in virtual machines with vSphere FT enabled.
VMware vSphere Fault Tolerance (FT) provides continuous availability to virtual machines that require a high amount of uptime. If the virtual machine fails, another virtual machine is ready to take over the job. vSphere achieves FT by maintaining primary and secondary virtual machines using a new technology named Fast Checkpointing. This technology is similar to Storage vMotion, which copies the virtual machine state (storage, memory, and networking) to the secondary ESXi host. Fast Checkpointing keeps the primary and secondary virtual machines in sync.