I totally agree with you on the two as a couple! They were very believable, and although I never forgot that Reese was Reese I still thought she did a great job.
Some friends of mine were disappointed in the movie because they wanted to see their WHOLE life story and wanted to see Johnny as a "real" Christian. I told them that I loved it because it was interesting and the rest of their life story was anticlimactic. Alas, some people want nice clean happy endings and these friends are just such people. Disappointing!