Anthony Head?? Giles from Buffy?! *dies*
OK, seriously...I don't know what to expect from this movie. I love the casting for everyone other than Sweeney Todd and Mrs. Lovett--Alan Rickman will be perfect as the Judge, Anthony Head rules in general, and the Borat guy (can't remember his name) should be really good as Pirelli. I've also heard that they've cast an unknown with a great voice as Toby.
However, I have my doubts about how well it'll really turn out. We all saw what happened when Tim Burton and Johnny Depp got together to try to make Charlie and the Chocolate Factory scary--that was just awful, and Depp completely failed to impress (though I love him in most things). Then there's Helena Bonham Carter, who from what I've seen pretty much only knows how to play one role--crazily creepy (see, for example, Corpse Bride, Frankenstein, and Harry Potter 5). I can't imagine her as Mrs. Lovett (who is crazily creepy...but in an old woman kind of way rather than an insane corpse-curse casting way...).
Russell Crowe was originally supposed to play Sweeney Todd. How I wish that had worked out!