You get slightly slower acceleration but about 10% to 15% better mpg. Bio fuel, because it 'explodes' slower it gives more power over the whole downward stroke of the piston giving more efficiency. Normal diesel explodes to fast which is why a diesel engine sounds 'clunk clunk clunk'.
One drawback though, sometimes when in traffic, it can stink like an old chip pan.
Originally, the diesel engine was designed to run on peanut oil and a diesel engine can run on any heavy oil - soya, rape seed, etc or any mixture including old sump oil provided it's been cracked properly.