Is actually not the first synchronized sound cartoon. Walt Disney's rival Max Fleischer had produced a series of sound Bouncing Ball "Song Car-Tunes", which featured synchronized dialogue and music starting in 1926. The first to be produced was My Old Kentucky Home (1926).
Before the copyright for "Steamboat Willie" expired was set to expire in 2003, Disney lobbied the US Congress successfully for an extension of copyright protection by twenty years. Because of this law, "Steamboat Willie" will not enter the public domain until 2023.
Variety's review of "Steamboat Willie" read: "Not the first animated cartoon to be synchronized with sound effects, but the first to attract favorable attention. This one represents a high order of cartoon ingenuity, cleverly combined with sound effects. The union brought laughs galore. Giggles came so fast at the Colony they were stumbling over each other." (Variety, November 21, 1928.)