I finally caught up with Cloverfield (Matt Reeves, 2008) on DVD. I thought it was effective and diverting, but that, for the most part, it's central gimmick - that the whole thing is meant to be "found footage" - never stopped being just a gimmick. The film would work almost exactly the same way without it. It does add a "conceptual" layer - i.e. it provides a topic for conversation and/or "think pieces" - but it's a surface layer: there's no depth to the concept built into the movie.
Not that there's anything wrong with that, but Cloverfield is weak sauce compared to The Blair Witch Project - a movie which I don't particularly like and didn't find all that scary, but where the same gimmick becomes more than just a gimmick because that choice - that the movie is meant to be "found footage" - influences every other choice made by the directors - acting style, cinematography, staging, temporal-spatial integrity, etc.
And as for the marketing angle: the guys who made and sold The Blair Witch Project were genuine independents going for a grassroots approach. That's certainly not the case with Cloverfield.