For sure the negative side of bio diesel is there - particularly palm oil (which I was using in the 'test mule' golf). But let's face it most of the oil we are talking about here is sunflower/brassica crop based and not originating where there are food shortages. And as we know on a global basis we are pretty crap at balancing areas of agricultural overproduction with areas where people are starving! So what I am trying to say is yes there are major issues with bio fuels, but I wouldn't lose sleep from burning EU cooking oil! My palm oil (originating from a large multinational that own a palm plantation that used to be natural tropical forestry, and in a country where malnutrition is a BIG issue) is a different story...
But, there are some positive sides to bio diesel - example being planting up of degraded tropical land with Jatropha which is extremely hardy, yields quite good oil and stabilises land which otherwise would be extremely marginal, further depleted and allow further desertification.