I managed to see "Sweeney Todd" opening night, and it was a rather odd creature.
"Sweeney Todd" is based on a Broadway musical and in many ways succeeds. One of the primary challenges in bringing any play to the big screen is fleshing out the world that the play inhabits. So much that can be left to the imagination on the stage cannot be on film, and with only a couple of missteps "Sweeney Todd" is well realized. London is effectively transformed into a dark and dreary Burtonesque nightmare. There were only a couple of scenes where the CG used to bring this about were obvious (one near the beginning where they forgot to add any shadows to the actors, one involving a quick trip down a tunnel which made everyone seem like a cardboard cutout, and one near the end involving a giant meat grinder), but you can't fault the rest of the production values.
As far as acting goes, you can't really fault any of the actors either. Sasha Baron Cohen was an inspired choice as the rival barber, and Johnny Depp truly became the Demon Barber of Fleet Street. I wish Helena Bonham Carter had managed to actually pick an accent and stick with it through the entire film, but her characterization was spot on.
There is one large glaring hole in this film, though. For a movie based on a musical, the musical portion of the film fails miserably. Neither of the film's leads can carry a tune in a blood-spackled bucket. To be quite honest, I think the film would have been more enjoyable had they simply transformed the music into conversations. Quite the shame, given how much I've enjoyed other musicals-turned-film in the past.
If you are a Depp or Burton fan, it's worth seeing, but beyond that it is an ideal example of how to fail when bringing a musical to another medium.