I've been running B99 in my 1999 E300D for about a month now (tax incentives mean they have to mix in 1% petrol). The car idles quieter and I'm also no longer seeing any smoke under any conditions (even on cold start). The exhaust smells a lot better as well.

The power seems about the same and so does MPG. It may be slightly less, but I haven't been on a good highway trip to tell yet. So far I've had a couple tanks run through the car and had the fuel filter changed just today as a precaution. Overall, I'm happy so far.

In my area Diesel and Biodiesel are almost identical in price. In fact petrol diesel is more expensive at some stations (Portland, Oregon). The economics of biodiesel are starting to be competitive and I feel good knowing I'm giving my money to local farmers and not mid-east oil sheikhs. That, combined with the significantly less environmental impact, makes it worth using.