*Freedom to Travel and work anywhere in EU : Membership in the EU is the main advantage. The best thing about it, in general, is the access to the free trade zone. By becoming part of the EU, any country has the access to the markets of all the other countries in the EU. This is generally an advantage because it increases the size of the market for each firm.
*Common Currency : No need for other businesses to exchange currencies because they will be paying and paid the same currency which makes the processes much easier for the mangers. "the euro, these exchange costs were estimated at €20 to 25 billion per year in the EU (as much as 0.3% to 0.4% of GDP)" - Economic and financial after.
*Economic : Economic development of countries like Portugal, Ireland and Spain. These countries made great in catching up with their other European countries. After the fall of Berlin Wall, Eastern European economies, such as Poland, Czech Republic and Hungary have also seen strong growth in real GDP.
*Border Crossing : Because there are no checks are carried out at the borders between countries it is not secure at all. The borders are free for all the EU countries and are free to go in and out which will lose the security.
The main disadvantage of being part of EU is not agreeing on something. For instance some countries would like to have or do things that other countries would not like to. Also, each countries has its own point of view and would like different things then the other countries. Disagreeing is the main disadvantage of being part of the EU.