Well, I was expecting I might get resistance particularly from our Western members. It could just be that I'm unfamiliar with how guys like that would really talk. The movie versions, as I said, I could totally buy, but to me the story guys seemed to veer into the realm of caricature. But then, I've never lived in Wyoming.
I've only spent a little time in Wyoming. There are subtle variations in the Mountain West dialects, I think, but my ears haven't gotten good enough to pick them up. I can tell a dialect from the mountains from one from Texas or from the midwest (Iowa, for instance), though. And the differences from Californian or Southern or New England accents are pretty extreme... though I probably wouldn't say that if I spoke British English.

But, ok, compared to the Colorado accents I know, I would say that the rhythm of the language is pretty good. There actually is a little bit of a break between the "gonn" and "a" that you don't hear in, say, rural New England. And if I listen closely, I can pick up the hints of two syllables, the "goin a" that's typical of the way Proulx writes the dialect. But, tell you what, it still looks weird on the page to me. It's just not the way that most American writers phoneticize rural dialects.
(And the description of Jack's Texas accent as he grew older... that's spot on, as my British friends would put it.

But I'm sure glad that Proulx didn't decide to write Jack's Texas accent phonetically, because that would have REALLY hurt my eyes.)
Aside: I think it's really, really hard to write American dialects, at least, in a way that doesn't seem to make fun of them. I mean, to the people speaking the dialects, that's simply the way the words are pronounced. Spelling them phonetically seems to say that "these people are speaking wrong."
And when TJ wrote with a deliberate accent, it looked really exaggerated to me, and I've heard enough Oklahoma accents that it wasn't just unfamiliarity with the dialect.
I'm less certain about how well colloquial expressions in BBM work. Those are the sort of things that can vary a lot from one place to another, in my experience, and that can get lost within a generation. And they might be the sort of thing that a 60-year-old man would not say in the presence of a 40-year-old transplant woman. (But would they say them in the presence of a 60-year-old transplant woman? How would Annie Proulx pick up the language that two native Wyoming men speak to one another?) At any rate, I've never heard anyone talk about "whipping babies."
One thing that I've heard other people (maybe at the Dave Cullen forum?) mention: all the swearing. Somebody somewhere mentioned that the ranch kids they knew tended to be really polite in their speech, even if their grammar wasn't perfect. And a friend of mine who teaches middle school to ranch kids has made similar comments -- that she's never met kids who say "yes, ma'am" and "no, sir" more than the kids who were raised on ranches.