What is your opinion regarding the use of bio-diesel ? Does it help keep the fuel & intake system clean ? Mileage does seem to improve, i'm assuming B5 or B2.

Bio fuel does act as a natural solvent, however at the levels of B2 or B5 it is doubtful that it will have much of a cleaning effect due to the low concentration. At those levels of bio fuel the fuel economy impact should be almost negligible. With concentrations of B50 or B100 the drop in fuel economy will be more noticeable in the range of 4-5% for B50 and 8-10% for B100. One good thing is bio is bio degradable and will not harm the environment if spilled on the ground or in the gulf of Mexico for instance.