I was happily working with Entity Framework 1 until I read at several places about its lack of important features that are normally expected from an ORM framework. I started thinking seriously about upgrading to Entity Framework 4 (yes, the next version after Entity Framework 1) after I read this No Confidence letter. It lists all the issues that EF1 has and also suggests improvements. Several architects signed it.
So I upgraded to Entity Framework 4. This required me to upgrade my projects to Visual Studio 2010 & .Net Framework 4. Along with this, I also upgrade my MVC 2 project to MVC 3 with the help of this article.
Although POCO & Persistence Ignorance are the two most-liked features, they did not interest me because I had already generated the Entity classes from the database in EF1 and mapped them to my own model. What interested me was Lazy Loading and Foreign key association.
Entity Framework 4 addresses several issues and it has undergone a major rework. Worth upgrading.