It depends on the lens. You have to make a research first. But most of the times very fast lenses are quite soft wide open.... Take as an example the new nikkor z 50mm f/0.95. It is insanely expensive and wide open is very soft. But when stopped down is amazing. But why buy an insanely expensive lens to use it at f 2.0 for example? When you can buy a cheaper lens with good sharpness?
From what I ve read the ef 50mm f/1.2 is an excellent lens! It is quite expensive though. The 50mm f/1.4 is 1/3 of a stop less bright it is very well built and cost about 1/3 of the f/1.2 version.
I believe that investing in lenses is more important than investing in cameras which come and go as time goes by and things advance. Lenses are a long term system investment and they do worth the money. The final decision is up to your needs photographically speaking.
It is a great thing to have a fast lens. There were many times that my nikkor f/1.8 saved me in low light situations. As well as my f/2.0 helios. A fast lens gives you always an advantage. And the nice thing is that when you shoot at low light and you want a bit sharpness you can just go for an f 2.8 and you are fine. The advantage of an f/1.2 is that you can go for an f/2.0 or an f/2.8 with an even better sharpness performance but paying a higher price. Also the faster and thus more expensive have most of the times more aperture blades that cheaper models that improves bokeh performance in spot light sources.
I use a lot my 50mm lenses and they impress me always! When you will get used in using this focal length (which of course is not suitable for all the cases) you will surely adore them.
Their simplicity in their optical formulas (this does not apply in all of the 50mm design cases) is their basic advantage.
And why do faster lenses cost more?
Well glass and optical formula design is one of the hardest things to beat in optic industry. As glass diameter increases the harder it gets to properly direct the light into the camera having to deal with glass properties such as refraction chromatic aberations lens coatings and many other things. And as glass goes larger the design costs boost up in a non li ear way.
I think a 50mm f/1.4 would be a great solution. The f/1.2 is surely the hi-end nifty fity of Canon and deserves its money. But check your needs also.
And I'll go and a bit further. We are humans. And as humans we have our dream objects. Our dream lenses or cameras. And I'm also such a person. And many times event hough I might not need the full potential of something I like, I will go for it.
Look. At the end of the day the ef 1.2 is a great lens. And surely you will not regret it performance wise.
Edit
You will notice an opinion bias change through my writings. This is because while writing I looked a bit on reviews on the 50mm f/1.2.
I prefer to present facts and points in a discussion rather directing somebody to make a decision. And this is why each one of us has its own point of view.