Well, I run pure B100 and I'm very satisfied doing so. Here's what I found out in 2 years on my 240D and my 300D as well as my Dodge Ram 4X4.
Pro's
1. Biodiesel runs much cleaner and I get through emissions with no problems!
2. Mercedes Benz and Cummins did a study from 1994 to 2000 comparing Biodiesel and concluded that it helps reduce operating costs because it is not as corrosive, it runs cleaner and after 6 years of metal particulate tests, it would not be unusual to be able to get twice the engine life from Biodiesel vs. Fossil fuels!
3. I do not have to wait in line for fuel and when I go to Western States Petroleum, I get enough to carry me through for a week or longer and just pump from my house.
4. The engine runs a lot quieter and with much less smoke.
5. The exhaust smells like food cooking! (It's amazing how many appetites I stimulate with both cars!)
6. If you spill some on the ground, it will not poison the soil. (Millweeds however love it, it will not kill them. In fact they grow taller and bloom more profusely, so if you have millweeds you want to get rid of pouring Biodiesel on them to kill them won't work!)
7. We spend the money on fuel here instead of sending it to other countries that don't like us. (Feed a farmer, starve a terrorist!)
8. You help the U.S. achieve alternative energy independence while helping out National Security at the same time.

Now the Con's
1. Your actual horse power or engine kilowatt out put will be a little less, but not enough for the small difference to become a problem. Expect about a 5% reduction in performance overall. If you burn Biodiesel at 100%, then you should turn up the fuel ratio about about 1/2 to 3/4 turn and adjust your engine timing by a 2* advance to compensate. You will still get through emissions with flying colors and you will feel it in your right foot.
2. Carry some spare fuel line hose with you in the trunk. Biodiesel doesn't like organic rubber and you will need to check your injector bleed hoses and change them more frequently.
4. Biodiesel gets a little cranky at colder temps and you going to have to mix with fossil fuel or kerosene to keep it moving depending on where you live.
5. It is not as easy to get as regular diesel, so you may want to store some at home. I have a 200 gallon fuel tank for this, but I live way out in the country. Use your own judgement. If your close to the source, a couple of 5 gallon jugs could work out just fine!

The Pro's outweigh the Con's for me and everyone I met feels the same way. I haven't really noticed the fuel economy difference everyone else seems to have, but your fuel economy could be a bit less. It is still worth it though.