They're an interesting thing to say the least. Finland had a lot of wood gas generators in use during WWII as what limited petrol we had went towards the war effort. Other substitute fuels were used a lot as well, e.g. pine turpentine which we had a lot of as it's a side product of refining wood into pulp.
It's a relatively efficient system, and burns quite cleanly in the engine (as soot and other contaminants are filtered out by the generator). Though they're quite dangerous, since wood gas is mainly pure carbon monoxide. The amounts they produce are so high that leaks or topping up the generator can cause carbon monoxide poisoning even when outside. In Finnish we actually call the generator häkäpönttö, which in English would be a carbon monoxide can (häkä colloquial Finnish for carbon monoxide, pönttö a colloquial Finnish word meaning a can, container, carton or someone dumb in a usually non-derogatory way – closest equivalent in English would be dummy).