The difference between f/1.2 and f/1.0 is virtually nothing in practice. Between the actual light transmission, the diminishing returns on apertures larger than f/1.4 and the vignetting which takes over more and more of the image, you're only actually gaining about a quarter of a stop in the center and losing a quarter of a stop from the mid-frame outwards compared to the f/1.2. If you stop it down to f/1.2 then you do actually gain about a quarter of a stop at the corners compared to the f/1.2 lens wide open, but then by f/1.4 everything is equal again. This is precisely why the lens was moved from f/1.0 to f/1.2 in the first place. Make no mistake about it, the 50mm f/1.0L was a fantastic lens at the time, but it has been bettered and by modern standards it is positively not a Noctilux. (Which actually does justify its price, optically.)
Given its not giving you any kind of optical advantage over the f1.2—and many would argue even that lens doesn't actually give you any kind of optical advantage over many higher-end but still cheaper 50mm f/1.4 lenses—and the focus is slower, plus wear over time, there's really no point buying a 50mm f/1.0L unless you're a serious collector and you've got enough cash to spare that you'll not notice the hit to your bank balance. As a photographer, it's a worthless relic. It might be fun to use once and show off a little, but in practical terms it is inferior to other fast 50mms that you can buy for a lot less. Myself, I used one a while back and I can't say I would pay anything for one. I'm not a collector; whatever I buy has to actually make sense as a working lens. To that end, the 50mm f/1.0L is only about as valuable as the current 50mm f/1.4 USM is. I'd gladly pony up the money for a Noctilux 50mm f/0.95, if the Leica bodies didn't cost so much, too. For anything less than that, I'd either just buy a new 50mm f/1.2L or, knowing I mostly shoot at f/2-f/4, I'd just buy a f/1.4 lens and pocket the rest of the money for things which will actually make a difference. Three grand for a 50mm f/1.0L is pretty absurd for anybody who isn't a collector with a high income.