We're evaluating different options for distributed cache. I have some experience with memcached but no experience with Velocity (App Fabric cache services).
I wonder if using App Fabric only for distributed caching make no sense taking into account that there are very mature tools as memcached specially designed for this purpose.
If you own the infrastructure and plan to continue owning the infrastructure for you application, then supporting the tool of familiarity makes sense. If you have the ability to manage a heterogeneous environment for the life of your application (say 5-10 years) stick with memcached. If you think you are going to deploy into Windows Azure, then you will only have App Fabric as an option.