Here's the formula for determining whether you'll like Joss Whedon's take on Much Ado About Nothing. If you like Joss Whedon, there's a good chance you'll like it. If you aren't one of the faithful, you may not; the widespread good reviews this is getting may be proving me wrong, but this feels Whedony enough to me that I could imagine those who aren't part of the cult being turned off by it. There's a further test, though; if you like Joss Whedon, I'd guess there's still a fair chance that you care enough about Shakespeare to have definite views on how he should be filmed, and then, maybe you like this, maybe you don't.
I'm part of the cult, I guess, and I do care about Shakespeare, but I think my standards for a Shakespearean production are probably fairly lenient. Basically, I want the actors to sound like they know what they're saying. I want the language respected, and for me that means I want it spoken like it means something. This film meets that standard, by and large; only Spencer Treat Clark as Borachio sounds overwhelmed by the diction. Everyone else delivers their lines so that you can sense the intelligence that wrote them, as well as the feelings of the characters who are supposed to be feeling them.
I think that's probably the film's strength: that it puts the language front and center. Even the decision to film in black and white, whatever motivated it, has the effect (for me) of deemphasizing spectacle (something the modern setting already does) in favor of language.
On the other hand, I didn't find myself laughing at most of the jokes in what is, really, a very funny play. I did laugh during this film, but more at the jokes Whedon inserts - little bits of non-scriptural business, camera work, etc. (what he does with the line "even if she were an Ethiope" is classic Whedon). For the most part I found myself appreciating the jokes, thinking about them, rather than reacting to them.
I'm not going to say this is necessarily a bad thing. But it does make me appreciate the Branagh adaptation of twenty years ago even more than I already did; for all its flaws (and it has a few), it approaches the humor with a naïve brio that makes it work more or less as I imagine it might have worked in Shakespeare's day. At least, I laugh at that film quite frequently.