I was browsing around (as you do) and came across this discussion (ooh err). Having used bio-diesel, I thought I'd come and share my experiences.
My boss recently started producing his own bio-diesel to run a fleet of about 15 trucks, from P-reg's ('96) up to a 54 plate ('04), (he couldn't use it on the 55 Scania cos of the warranty - check with your manufacturer before putting it in anything of value)
It cost him thousands to set up - you need several filters to clean the stuff up, then there's all the pumps etc. (not to mention his mug of a nephew that spent all day in there making the stuff)
I don't know exactly what he added to it (but I think it was meths and ...something else), but it needs something to actually produce the 'bang'.
As it was recycled stuff from chip-shops and pubs (with the duty, buying it fresh would cost more than diesel), it smelled. When I say smelled, I mean go stick your head in a chip fryer that hasn't been cleaned for a month, then multiply that multiple times and you're starting to get close.
From an environmental point of view, you can't get much better (electric just won't cut the mustard if you're pulling 44ton!), all of his oil was sourced from places that had to get rid of the stuff anyway - this way an otherwise useless by-product (of chips), was converted to tractive effort and exhaust fumes - something you're going to get from these vehicles anyway, at least veg oil is cleaner than diesel.
Effects on the vehicle? Increased fuel consumption, but more power. The only real issue we had was fuel filters - they needed cleaning every month and replacing every 3-4 months, and in the cold it had a habit of waxing, but a mix of 50/50 bio/diesel solved both of these issues.
Also for anyone with boats or caravans - it'll bugger you're night-heater (a little diesel burner to keep you nice and toasty when the engine's off), but I guess most would be running them on red(?), so I guess that one only really affects the truckers.
In summary? Only economically viable (doing it 'properly' that is) if you're supplying several vehicles. And don't use it if you like chips - I've been put off for life!