I saw one at a friend's house last week. It was much larger than a domestic boiler - I'd guess at 4x to 6x the footprint and about 1.5 to 2x the height. The unit was imported from the US at a cost of €10k. It was running when I was there, and there was no smoke/fumes evident - the oil tank area was very messy though, I think that the operation of topping off the storage tank with small drums would make this unavoidable.
I would be concerned about the supply of used oil - it is becoming increasingly cost effective (and environmentally responsible) to reprocess used lubricants, and I can forsee the day when garages etc. will be required to account for waste oil disposal.