While Microsoft has been telling us over and over how multi-server management will be this great way to manage all your instances, don’t plan on managing your SQL Express instances with it. Multi-Server management will only enroll Web Edition up through the Data Center Edition. The Express edition will fail to enroll within the feature.
Also, if you planned on using the dashboard views and drilldowns or the policy based resource utilization evaluation then you better pay up. These features will be a datacenter edition only feature.