I find LuxRender very easy to use. The learning curve is in using the exporters. all you have to do is create your scene in your 3d software and then export it to an lxs file. Then you open up the lxs file in LuxRender and you let it render.
The nice thing imo other than that its open source and "free" is that lux uses physical materials, I found that once I adapted to the idea that lux was a simulator and that all the materials had a physical correspondence things became simple, I was able to predict how materials behaved using physics not a abitrary "looks real" standard. At this point I doubt I would be able to work with a non-physically correct renderer. In the end I have to say physical materials have given me a lot of freedom as a artist and there is a connection with real life art, much of what I do in lux I think could be implemented in real life.
I think lux is in fact simple but you just have to get used to it.