Use it for frying chips if you want but I use it in my old Land Rover Discovery (300tdi) as diesel replacement. Some may run fully on veggy oil but I just tend to put say 15 lts in when tank is half full of proper diesel.
Engine runs fine with little noticable difference. Perhaps not quite as much pulling power when warming up but no problem for my type of driving. Less than 70p a litre compared with proper stuff at £1.15.
But beware. Not for your modern sophisticated diesel engines, BMW, Bentley, etc. But seems fine with many of the older models. Search the net if in doubt.