It works all the time. I have a 9x9 plot of Netherwart/Potatoes/Carrots/Wheat, and then another 9x9 plot of 4 different types of trees, serviced by 2 planters, 2 harvesters, 2 fertilizers, 4 sewers and 4 composters (each sewer fed by 24 cows), and a separate plot of pumpkin with just a harvester. This produces *thousands* of materials for the Bio Reactors, which can each power 10 Bio Generators. My current setup has 40 active generators, and can power my EE at full (500 MJ/tick) with my AE system, various support machinery, the farm itself, and a quarry. If I need more power (f.ex. when I update to 1.5.1 and have mining lasers) I will just add more generators/reactors as needed.
Honestly, Biofuel is pretty OP and could stand to be nerfed a bit.