First and foremost: I knew that both Johnny Depp and Helena Bonham-Carter were at least capable of singing, having heard them in Corpse Bride. Now, Angela Lansbury's Mrs. Lovett was so perfect that I didn't expect any actress, much less Bonham-Carter, to come close, and I was right there, so I cut her some slack. She could hit the notes but didn't put much excitement into them, especially during her opening scene. She did pull off most of her humorous lines well, at least.
But the part of Todd demands a powerful, imposing vocal performance, and Depp simply didn't cut it for me. Not in singing, not in acting. The constant brooding look was good but the voice did not match, and the affected British accent only distracted from the content of his lines. Plus, even with the makeup, Depp had such a youthful appearance that I couldn't believe he had just come from fifteen years of hard labor in an Australian prison.
Next: Anthony and Johanna. Both could sing well, but their big duet (actually quartet, with the judge and beadle) was cut. Other omissions from the original script made it hard to understand the relationship and attraction between the two.
The other characters: Sascha Baron-Cohen only got to sing the first verse of Pirelli's song, which he did reasonably well, though in a lower register for most of the song than I expected. The second verse was also omitted in the newer stage version we saw recently, but that's unfortunate as that verse is funny and important for character development. Perhaps as an actual child was playing Toby in the movie, the director did not wish to show him actively abused as he is during that song in the stage version. The child actor playing Toby was excellent, and got to sing most of his original lines. The Judge's song about Johanna was cut, thus eliminating another important selection for character development. The Beadle barely got to sing at all, but his is one of the most vocally demanding parts in the original show.
One real problem in adapting this musical to film is that the chorus is omnipresent in the stage musical, and doesn't really fit in well on the big screen. I could understand doing The Ballad of Sweeney Todd in instrumental form only, and not singing The Letter (though it was shown word-perfect on the screen), but with these choral parts gone the flow was very different. Some scenes, like By the Sea, were well-done and even worked better in the film than on stage. But these were few and far-between.
The gore: I squinted during all the throat-cutting scenes, till the end when Sweeney met his demise. Then I saw that the blood looked exactly like tomato sauce and was really nothing to be afraid of. The bits of body parts were gross but not nauseatingly so.
Besides the hilarious By the Sea scene, the only thing I truly enjoyed about this film was the desaturated color look throughout, which well-illustrated the depressing grittiness of turn-of-the-century London. Here again was where a big-budget film could provide the kind of realism a stage production could not.
My conclusion: This film is worth seeing, but barely, and really only for the Sondheim score. If you've never seen Sweeney Todd in any form, I encourage you to skip this movie and rent the film of the stage production with Angela Lansbury instead. Or at least watch it after seeing this film, to fill in the gaps and hear the rest of the great music.