I've used them a lot. Size of a small fridge on wheels. They produced the first softboxes, called Fish Fryers for reasons that always escaped me. Weighed a ton with heavy perspex diffusers, slung off a small gantry. Heads were fed by cables the thickness of your arm. Rubber soled shoes are advisable. Even better, get your assistant to make the settings for you. Yes, popular with still life advertising in the 70s and 80s. Awesome things that impressed clients no end, especially when they went off with quite a thud.
They had a lot of power for the day, which you needed for those big, inefficient Fish Fryers and f/64 at 50ASA on a 10x8in large format camera. But bear in mind that 5000J is an electrical rating that doesn't translate directly to light output, and it certainly did not in those days. I recall using multiple flash quite a few times to get really high f/numbers.
They were mega expensive and de rigueur back in the day. I suspect they are now the kind of things that you have to pay someone to dispose of for you. I wouldn't even think about it.