One slight corrective to some of the comments here, German units were used sporadically and not in great numbers on the Italian Front. Austria-Hungary only really needed German help there in 1917. In 1917-18, the British and the French both sent substantial forces to the Italian Front, if I remember correctly they outnumbered the German reinforcements, so these roughly cancelled out.
The Italians sent forces to other fronts, but you really have to try hard to argue that they made any difference anywhere. For Austria-Hungary, Italy was the main front, but they weren't exactly effective elsewhere, and logistics, that bane of amateur military strategists, would have precluded ideas such as sending the entire Austro-Hungarian army to Ukraine or something. Actually the idea that Austria-Hungary somehow lost the war for Germany, other than starting the war in the first place, is not clear cut, they were able to handle their complicated strategic situation, they just kept needing German help, but the German forces spent could always be spared from the Eastern and Western Fronts at the time (OHL was also good at juggling). No one has been able to point to a battle where the Central Powers would have won a decisive victory if only those German units hadn't been sent to Serbia/ Romania/ Galicia/ Italy.
Where Italy's entrance to the war did make a difference was on popular support for the war in Austria-Hungary. It was the only event that made the war sort of popular. That it was the only front where Austria-Hungary fought effectively was no accident, pretty much everyone agreed on teaching the Italians a lesson. The territorial concessions that would have been needed to keep Italy neutral would have also hurt morale.