I have been using Yafray for some time now and I have been also sampling other GI renderers. I came accross a new one lucille here-
http://web.sfc.keio.ac.jp/~syoyo/lucille/index.html
I seems to be slower than Yafray, but it imports Rib files which is not bad considering that we will be getting rib file export for Blender.