Psychologically they had a great impact (a huge mass materializing out of the clouds in the dead of night with an ominous drone and indiscriminately sewing death and carnage below it is usually good for sapping morale) but militarily didn't have much of an impact, especially not on the Western Front- British and French defenses caught up with them pretty quick, and the usually dismal weather over England and the Channel made them less than reliable as precision weapons.
They were more effective on the Eastern Front, where defenses were much less well developed (they made several devastating raids in Romania (especially on Bucharest), the Balkans and the Dardanelles) and weather was generally more favorable.
