Well, depends on your idea of "aircraft". Zepplins were used heavily during the Russo-Japanese war. Now their were balloons before that time, going back to the late 1700's, but I'm assuming that's probably not your idea of an aircraft. The first actual plane use was during the Italo-Turkish war, with spotting and bombing runs against a Turkish base at Ain Zara, Libya.

There were also many actions during the First Balkan war. The Bulgarians and Greeks were religious users of planes, creating the first co-country bombing raid against Turky, and the first night raid.

During WW1, aircraft were put into conflict immediately, providing a great advantage for artillery, giving them the exact positions of troops. A British plane dropped a bright green light on a German trench in August 1914, and they were all confused. A few minutes later, the British bombed the crap our of them with artillery. The aircraft provided the exact position of the soldiers. So needless to say, aircraft were pretty successful.