The first thing people need to know about Sweeney Todd is that it is based on a musical. DO NOT complain about this film and give it a low rating because you didn't realise - the Broadway musical first came out in 1979 and was a huge success. The music and lyrics, written by Stephen Sondheim, are brilliant.
Tim Burton did a fantastic job of adapting Sweeney Todd to a film - he trimmed a complex 3 hour musical to a 2 hour film, in which the story was still completely coherant. The casting is perfect, where all characters from the musical are played by great actors who end up absolutely owning their respective roles.
While Depp's singing voice is not perfect, he plays Sweeney with a fantastic dark and brooding manner. Helena Bonham Carter's singing is great, considering Mrs Lovett's is one of the hardest roles. Her acting is, as usual, spot on. Alan Rickman and Sasha Baron Cohen, both incredible singers, also steal all scenes they are in.
Burton has taken a great musical and made it in to an even better film, with a suitably dark tone, brilliant sets and, as I have said, a stellar cast. This is easily his best film since Nightmare Before Christmas, and one of the best movie musicals of all time.