Well, Mussolini's choice to be neutral and later conduct independent wars probably saved his regime and his country the total war that developed later on. Germany obviously lost out on the manpower and means to attack the British empire, but gained by having Italy provide "volunteers", equipment, and supplies for the Russian front, just like Spain.
It's likely that the Italians would have given the western allies some practice before their initial disasterous invasions attempts, as the average Italian division was not as good as the troops sent to Russia. Their record in Greece is a good indicator of the Italian army's preformance in general.
Who knows? Maybe Russia would not have experienced as much damage as they did if the Germans were preoccupied fighting with the Italians on other fronts. Maybe more allied soliders would have survived if they had combat experience outside the initial invasions of Norway and France. Ultimately I doubt Italy would have been enough for Germany to win the war, but then we wouldn't have the entertaining experience of watching Mussolini's grandchildren in the Italian parliament today!