I'm not Martin, but let me try to answer anyway
Yes, there is a lot of difference between 0.5 and 1. However it depends on the level of detail that your render produces. Depending on the distance of the camera to - let say - a rock surface, you may not be able to distinguish certain rock details anyway.
Usually for stills, it's well worth to increase the render quality to above 0.5 in the final render. My personal preference would be to use 0.8 as the minimum for final renders. I'm actually using 0.8 fairly often. Up to 0.9, and if the scene details requires it, I also use 1 every now and then.
For Martin's jungle render, I reckon, 0.9 would have been sufficient IF he had used the soft AA filter. Could have saved him a few hours of render time. Now that we have a choice of AA filters, they really have a big impact on the visible detail in the render. Martin prefers the Mitchell-Netravali filter, probably because it somewhat brings out detail even stronger. As a matter of personal taste, I prefer the soft filter for renders with a lot of distant vegetation - reminds me more of how photographs look like. That filter however "destroys" some detail in the render, hence decreasing the render detail from 1 to 0.9 in the jungle render would have been acceptable.
However, once you do have a lot of potential detail, 0.5 is definitely wasting lots of it. At the very least, use 0.7.
Cheers,
Frank
