Well, for one, Europe finally uniting under one banner is probably the best thing that could happen on this miserable rock, i mean, how can you stand against something that will see the worst disasters of Humankind to date, the First and Seconds World Wars, a thing of the past never to be repeated...
...Surely, there will be no 'United States of Europe', for, as has already been said, many countries won't stand for it (namely Britain & France), for should Europe unite ala the US, each country will be giving up the independence and national identity they have fought so hard to preserve over the past several Thousand years or so....
...Best case scenario; in another attempt or so, a Constitution will be created and the EU will unite a damn good portion of Humanity in peace, and will be a new step towards a Global Identity for Humanty. Also, if this happens, the EU will likely become a Global Superpower overnight, creating a perfect balance of SuperPowers that is need on this planet; the current SP - the USA, the soon to be a SP - China, and the EU. Reviewing history, one can plainly see that one must have three superpowers to be balanced. One superpower leads to that power becoming very much like the US is now and has been since the collapse of the Soviet Union, which has really been discussed to death so i don't need to go into why it's a problem. Two Superpowers will lead to a rivalry situation, namely that of a Cold War, which is a bad, bad thing. Three SuperPowers will create a Balance, like the old Shakespearian story of King Lear, a kingdom divided in three will have balance, in two, it will have chaos...
...So, really, the EU is all but inevitable, which is definitely a good thing Globally Speaking...