One of the challenging things with current version of Entity Framework, its leakage of testability. Which means when you build an application or module (e.g. Repositories) that depends on entity framework, it will be very hard to unit test your code isolated from Entity Framework. This might lead to conduct Integration Testing which will require a connection and interaction with underlying data store (database).
You might think of using Mocking to mock or fake Entity Framework data store dependent calls. But sadly Moq and Rhino Mock mocking frameworks do not support this. And you’ll end up hitting the wall. Both frameworks are great useful and I personally use Moq for a while now and very happy with it. But no one can deny limitations when exist.