One word summary: OMGSQUEE!
You want more? Okay, three word summary: Really. Frelling. AWESOME.
However, I realise there is a lot more that I could say in far more coherent terms, so here is something resembling a review, though I've forgotten most of what I was going to say.
Firstly, it is a highly condensed version of the whole musical, the most notable omission being the ensemble piece, "The Ballad of Sweeney Todd", which was recorded but cut for time. Nonetheless, the omissions don't look too obvious and the whole thing flows very nicely. The sub-plot of the young lovers is trimmed quite extensively, but in a film as dark as this, it would only have jarred, I think.
Okay. So. First things first. Burton's direction is amazing, as ever. As Empire mentioned, even the historical inaccuracies of his vision of London can be completely glossed over, because this is Burton's gothic fantasy version of Victorian London and it works beautifully. Burton's London is dark and depressing, lacking in sunlight and monochromatic, the perfect lair for the vengeance-ridden Todd and money-hungry Mrs Lovett... Oddly enough, the one colourful sequence in the film, aside from being hilariously funny (the striped bathing suit killed me dead on the spot, as did Depp's expression throughout), screams Burton more than the rest of the film.
Perhaps the thing I liked the most was the fact that it didn't look like a musical, at least in the traditional sense. No Rogers and Hammerstein here, no rubbishy songs-on-stage like Chicago, no big group dance sequences. It looked like a melodramatic Tim Burton gothic fantasy, with some songs thrown in for good measure, and that's exactly how it should be done. Instead of playing up the fact that it was a musical, it didn't even acknowledge it.
Oh, and he's made a return to his molten-jam B-movie blood like in Sleepy Hollow, providing the only colour to the majority of the scenes.
Okay, I could go on forever and ever about the prettiness, but I'll move on to the cast.
Johnny Depp and Helena Bonham Carter are brilliant in this, but they are well-supported by Alan Rickman, Timothy Spall and Sacha Baron Cohen. And yes, they can all sing. Really well, as it turns out.
There was a bit in the opening sequence when Depp sang-growled the words "you will learn" to Anthony where my spine turned to jelly, at which point I just sat back and enjoyed the ride. I didn't doubt that Burton made good choices (they had to audition, we've all heard that) but I will admit to being pleasantly surprised. It's a difficult score to sing, and both the leads did amazingly well with it.
Helena Bonham Carter has a beautiful singing voice, as it turns out, not unlike Nicole Kidman in Moulin Rouge - a pure soprano which is clearly untrained, no forced vibrato, just natural. Even more impressively, her voice meshes wonderfully with Depp's during their duet pieces, and both have retained their impeccable comic timing during the lighter songs. Alas, Anthony Stewart Head's role has been cropped to a very brief cameo after the shaving competition scene, and he doesn't have a chance to show off his own voice (he would have been part of the ensemble piece which was cut).
Johnny Depp has recycled his Pirates/From Hell voice for Sweeney, but it's easy enough to see through it, even though the opening scene on a boat does nothing to dispel the image of a slightly more disturbed Jack Sparrow (good crossover fodder, certainly); also his flashback portrayal of Benjamin Barker reminded me of Finding Neverland. There was a flashback sequence in particular, where Mrs Lovett is describing the fate of Mrs Barker, which made me yearn really badly for a Burton-directed version of Phantom. I knew it would have worked. :P
One final thing I should mention is the casting of Toby as much younger, which confused me slightly at first, as the character is usually 20-something. In the production I saw with Jason Donovan, the character of Tobias was a nutter from the local asylum, or at least that's the impression I got. I don't know if that's the norm, but it definitely added some interesting depths. Anyway, in ths movie Toby is a boy from the workhouse, making the already slightly dubious relationship between him and Mrs Lovett all the more eerie and bizarre.
I won't spoil the ending for anyone who doesn't know the plot (not that there aren't enough spoilers in here already :P) but it's a deserving finale for all involved. Well, perhaps not all...
In short, it's absolutely amazing. I went in knowing I was going to enjoy it, and knowing it was going to be good, but I had absolutely no anticipation that it would be that good. The kind of film where you come out having been totally blown away by the sheer genius, which I suppose in some way can also be attributed to Sondheim himself, and certainly one which warrants re-viewing. I want to see it again purely so I can focus less on the superficial brilliance and look for the subtle nuances and references (the blood in the opening credits trickled into a spiral shape, but I'm sure there must be more), and also because I enjoyed it so thoroughly that I just want to see it again.
A very, very satisfying evening, all told.
Everyone go and see it. You will not be disappointed.