OUR LADY OF 121st STREET is less a play than a series of blackout sketches and character studies. But oh, what sketches, and oh, what characters! Dramatist Stephen Adly Guirgis (Jesus Hopped the ''A'' Train) has set his piece in a Harlem funeral home, where the former students of the deceased Sister Rose have come to...well, not pay their respects so much as rip through a succession of rants, riffs, confrontations, and confessions that are either hilarious, shocking, or hilariously shocking. Guirgis has been blessed with a fabulous command of language, a uniformly excellent cast (especially David Zayas and Ron Cephas Jones), and a director -- none other than Philip Seymour Hoffman -- who doesn't seem to mind that the story doesn't make a whole lot of sense. Neither should you.