The Spanish site History of the European Union and European Citizenship (http://www.historiasiglo20.org/europe/index.htm) illustrates in a simple yet comprehensive way the evolution of the concept of citizenship in the European Union. Many links and citations are provided too, thus being a very good source for documentation.
I have found of particular interest the article Citizenship and Identity (http://www.historiasiglo20.org/europe/ciudadident.htm). Hereby an excerpt from this paper:
The Citizenship of the Union is not a consolidated reality, rather, we are attending to the beginning of a long process that will result in one or another way depending on European integration process fate. To fully develop a meaningful European citizenship is necessary that a sort of European identity arise
At the moment, the result of this attempt has been rather disappointing enough. Europeans largely ignore the new citizenship statute, the lack of information is the general rule, and, arguably, the feeling of European identity has not spread so much.
Which are the reasons of this relative failure? Answers, evidently, vary according to the previous views on European integration process.
For the most pro-Europe observers, the Citizenship of the Union, as it is detailed in the Treaties, is a completely insufficient legal statute. [...] the rights yielded by the Citizenship of the Union are quite limited, and the most important, free movement of persons, is not completely developed.
Conversely, there is another opinion, the eurosceptic that consider the advances as excessive and try to put a brake to any subsequent evolution towards political integration an full European citizenship. This view has its strongholds in Britain, especially in the Conservative party, and in Denmark.
In Denmark, the Treaty of the European Union was rejected in a referendum held in June 1992. The Folketing, the Danish Parliament, presented in December to the Edinburgh European Council, a document, Denmark in Europe, that summarised the reluctant Danish view about the advances of the Treaty of Maastricht, and among them, the institution of the Citizenship of the Union.
The Danish delegation approved an unilateral declaration that summarised a great part of the elements of its eurosceptic stage regarding the European citizenship:
" (...) the citizenship of the Union is a political and legal concept that completely differs from the conception of citizenship in the sense that it appears in the Constitution of the Kingdom of Denmark and the Danish legal system (... )
Any disposition of the Treaty neither implies nor foresees a commitment that binds to create a Citizenship Union equivalent to a nation-State citizenship (...)
The citizenship of the Union doesn't grant at all to any resident of another member State the right of acquiring the Danish citizenship or any other right, duty, privilege or advantage that come from the the Danish Constitution and laws."
The future of the Citizenship of the Union will depend on the evolution of the public opinion of its members States between these two opposed views.