Their nations were allied, but what did they personally think of each other? Is there anything based on documentary evidence that could give us some insight here?
Their relationship changed a lot through time. The following observations are based on Alan Bullock's work, Hitler: a Study in Tyranny.
Mussolini came to power in 1922, when Hitler was still basically a Munich gangster. Hitler wrote about his admiration for Mussolini's Italy in Mein Kampf... presumably Mussolini didn't even know who Hitler was at this point, although the Beer Hall Putsch made a big splash in the papers at the time, so I suppose it's possible.
When Hitler came to power, Mussolini was allied with Britain and France. When Hitler tried to take over Austria in 1934 (assassination of Austrian Chancellor Dolfuss) Mussolini actually moved troops up to the border to deter him.
It was actually Britain and France which forced Mussolini into Hitler's arms. When Mussolini butchered the population of Ethiopia, and Britain and France refused to help him do it, he went into alliance with Hitler instead.
They had a mutually helpful relationship from that point, collaborating over the Spanish Civil War, etc. When WW2 actually began, Italy hemmed and hawed before joining in. Hitler grew more contemptuous of Mussolini as Italy lost every major engagement with the Allies, except for the defeat of the British in Somaliland.