Any experience or thoughts about Stormpath and their like?

Just ran across Stormpath [1] in a recent mailing [2]. They provide a hosted user management service instead of you having write one (using Devise and CanCan, for example). Has anyone used them or anything like them? What do you think about off-loading your app's authentication and authorisation to a third party?

[1] https://www.stormpath.com/
[2] https://www.stormpath.com/blog/advanced-user-management-ruby-rails-and-sinatra