She's great in The Song of Bernadette, good in Portrait of Jeannie and fits those roles perfectly. Both films also have brilliant cinematography, especially the latter which has dreamlike atmospshere.
I agree she wasn't that great as Ruby Gentry but I didn't think she was bad either...just average. I'll watch next up Duel in the Sun if she's more credible there.
It seems to me that Jones is great as a saint but not so much as a devil, despite her shapely figure. But that doesn't stop me watching Duel in the Sun right now, rather the opposite! ;-P
Edit: Finished Duel in the Sun - and I think JJ was actually pretty good in it as lively half-breed. My final conclusion is that one shouldn't draw conclusions on Jones' acting abilities based on Ruby Gentry alone - where she wasn't at her best.
As for Duel in the Sun as a film... it was rather average and soapy, but with good performance from Jones and a great final scene.