The scale of the expansion is unprecedented, and at a stroke it will end the division of Europe brought about in the last century by war and ideological rivalry -- not that the theme of European unity, or at least harmony, is new.
And beyond the new borders of the expanded EU, yet more countries are seeking membership.
Romania and Bulgaria are already negotiating their entry, which is expected in the next few years. Turkey is also recognized as a candidate, but its accession is not assured.
Beyond them, Ukraine, the Caucasus republics of Georgia, Armenia, and Azerbaijan, as well as the western Balkan states of the former Yugoslavia, plus Moldova, and possibly even Belarus, are potential candidates.
But some analysts see the expansion process as having only a limited future, given the difficulties of coping with 10 new members. Peter Zervakis of Bonn University's Centre for European Integration sees it this way.
"The 'Europeanization' of the 10 new members has still to be achieved, and when we see how long it took to assimilate by comparison easy cases like Spain and Portugal and Greece -- more than 20 years to bring those countries up to a moderately acceptable standard, so that they can meet EU norms, and becoming functioning members of the community -- then you can imagine how long it will take [with the new members]," Zervakis said.
Zervakis says much thought is being given in Brussels as to whether the model of expansion being used now is appropriate for use in the future. There are fears that the EU could become a victim of its own success.
"It's clear that the European Union needs stable neighbors, particularly in the east and south,” Zervakis says. “And to achieve these stable contacts, new ways must be found which are not based on the normal plan of expansion."
The alternative under consideration is "privileged relations" -- a concept that is already incorporated into the proposed new EU constitution. It envisages membership for certain countries of the EU's huge internal market, but without political integration.
At the same time, it would mean those countries enjoying "privileged relations" would have to meet the rules, regulations, and standards of the EU, including democratic and human rights norms.
How would this affect the hopes of the new neighbors?
Among the small western Balkan states, Croatia has already submitted its application for membership to the European Commission. A decision by the EU on whether to open negotiations could come as early as June.
Macedonia has also submitted an application.
These two states would therefore seem slated for full membership.
But if the "new thinking" on alternatives to membership prevails, the other, less-focused former Yugoslav nations -- as well as Albania -- might have to be content with eventually becoming privileged nonmembers. That decision is some years away. Ukraine and Moldova would probably fall into the same category.
Turkey remains a controversial case. Although already formally acknowledged as a candidate for full membership, when that will happen -- if ever -- is not clear. French officials recently suggested it will not occur in the foreseeable future. Germany, on the other hand, supports Ankara's application.
German Foreign Minister Joschka Fischer believes it is important to have a moderate, democratic Muslim country in the EU because it would dispel the perception that the EU is a club limited to Christian nations. It would also give the EU greater authority in its dealings with the Islamic world.
Russia has never seriously been considered for EU membership, either by Moscow or Brussels. Many consider it too large and too different to fit in, and it is still inclined to cling to its old role as a superpower.
But this week, in the run-up to the expansion, Russia and the EU signed an agreement extending their existing Partnership and Cooperation Agreement (PCA) to the 10 new member states.
The agreement deals in large part with trade issues, assuring Russia of continued access to markets in the new EU members, most of which were once satellites of the Soviet Union.
Irish Foreign Minister Brian Cowen said the agreement would put EU-Russian relations on a “new level.”
"I believe that the EU and Russia can look forward to a productive summit in May and to bringing EU-Russia partnership to a new level. The extension of the PCA to the 10 acceding states would allow the enlarged European Union and the Russian Federation to benefit from the opportunities created by EU enlargement," Cowen said.