This topic has been discussed at nausea.
There are many criteria to judge the success of the Euro. Among some of them:

1. If the Euro success for the French is that the French team wins it, then we are doing good so far.
2. If the Euro success is that stadiums are full, that is an very low criterion as all seats had been sold long ago as usually happens for all Euro cups.
3. If the Euro success is measured by the attendance in fan zones, that is disappointing as expectations were much higher.
4. If success is measured by the number of tourists in France during the competition, that seems a disappointment by all indicators, including hotel occupancy and cancelled bookings or light flight loads.

The last criterion seems most important and results are disappointing. The Euro was supposed to project a great image of France, but when you see what images are shown on foreign media, it is easy to realize that it is a failure.