I meant to mention this a couple of weeks ago. After reading the book, I wanted to see the film, and did. I'm talking about the 1949 one, with Broderick Crawford as Willie Stark.
Three-word opinion: it's pretty good.
Slightly more expansive opinion: it doesn't do justice to the book, which of course you wouldn't expect it to. Apart from the fact that movies pretty much never can do justice to a good novel, this one is dependent on the narrator's introspection to a degree that would be difficult or impossible to transfer to image and dialog. And it either leaves out or changes a lot of very important things. But that really can't be avoided if you're trying to fit the book into two or three hours.
Still, taken on its own, it's a good film. I only knew Broderick Crawford as the hero in the old TV series Highway Patrol, but he's very credible in this role. John Ireland is okay as Jack Burden, but the character is pretty reduced in the film. Likewise for Joanne Dru's Anne Stanton. The real standout character portrayal is Mercedes McCambridge as Sadie Burke, Willie Stark's political guide and gadfly, and also his lover. She's perfect, even down to her physical appearance.
I was very grateful for one way in which the filmmakers departed from the book: they made no particular effort to place it in the South, and did not make their actors attempt Southern accents with the preposterous results that used to follow from that effort. Still does sometimes, but actors have gotten a lot better, especially those amazing British ones. The location is in fact California, but could be any area of rural 1940s America.
I don't know about "very great." That's Sadie Burke to the left of the big title card.
The DVD included some promotional stuff for the 2006 adaptation starring Sean Penn does apparently set the story in the South, and judging by the clips it doesn't work very well. Sean Penn's accent is not terrible but it's not good enough not to sound false. I'm not in any hurry to see this one though I may eventually, just out of curiosity.