Love the current answers, especially Catherine's.
Here are the first few lines of the Lord's prayer (often used to compare languages) in French and Italian:
Padre nostro, che sei nei cieli,
Sia santificato il tuo nome.
Venga il tuo regno.
Sia fatta la tua volontà, come in cielo, così in terra.
Notre père, qui es aux cieux,
que ton nom soit sanctifié,
que ton règne vienne,
que ta volonté soit faite, sur la terre, comme au ciel.
Note that written comprehension is better than aural comprehension. The two languages have very different pronunciation rules.
example: Italian "cielo" is roughly pronounced as: TCHEH-lo. French "ciel" as: syell.
If the two speakers have an idea of what is being discussed, guessing is easier.
None of those four languages are mutually comprehensible to a large degree (which is why they are labeled as languages and not as dialects).