I was asked about VMware EVC the other day and the advantages and disadvantages it may have. I had not used Vmware clustering with only using ESXi free and so was stumped at this question. Naturally I had to find out the answer. EVC alleviates the issue of vMotion compatibility between hosts that have different CPU generations in a cluster. EVC automatically configures server CPUs with Intel FlexMigration or AMD-V Extended Migration technologies to be compatible with older servers. As with almost all things Vmware you need to ensure your hardware is on the Hardware Compatibility list to ensure you are running supported EVC CPU Types. You can search the Hardware Compatibility List to verify if your processor models are listed.
There is also the restriction the EVC does not allow for migration with vMotion between Intel and AMD Processors. Other points you need to be aware of is the BIOS settings of these hosts need to Enable Hardware Virtualization and Execute Protection.
It appears that the enabling EVC in your organisation will have benefits in the long run however planning is critical to ensure ensure EVC will work with your current hardware, a planned maintenance is avialible to configure EVC since all virtual machines need powered off (or start with an empty cluster), the Bios settings of the hosts are correct and that if you do have a mix of Intel and AMD you are aware that you cannot vMotion between the 2 different types.