I've had an EF 50mm f/1.4 USM for about 13 or 14 years and it was used when I bought it... I have no idea how old it really is or how much use it got before I bought it. It's still working just fine.
Works fine and is nice and sharp at f/2 or f/2.2. My copy is usable wide open, only slightly soft. I considered getting the original Sigma 50mm f/1.4... but found the Canon sharper from f/5.6 and smaller. The Sigma was sharper wide open, but not by much. And I didn't see much difference between them from about f.2 to f/5.6. The Sigma also was more expensive and a lot bigger and heavier. I have never compared it with the "art" (which is ridiculously big and heavy). I don't understand the earlier comment suggesting issues with focus accuracy or speed. Mine is plenty fast and accurate.
Yes, as some others have observed in earlier responses, the EF 50mm f/1.4's AF system seems to be damaged too easily. Apparently it's a "hybid USM". It can be damaged by a hard bump on the focus ring or on the front barrel when it's extended for close focusing. I've never had any trouble with mine... possibly because I've always used it with the matched lens hood. That came with the used lens, which at least suggests the previous owner had done so too. It also seems the problems run in "batches"... Lensrentals.com buys a lens model in quantity for their rental business and they saw a high failure rate with one batch, but not with others. Of course, rental lenses probably don't get used as carefully as we treat the lenses we buy. Plus rental lenses spend a lot of time bouncing around in the back of a UPS truck.
With the 50mm f/1.4 USM the matching lens hood seems to be an important part of the solution. It protects the front barrel when shooting with the lens. Retract the barrel by "parking" the lens at infinity before storing it (which just makes sense with any lens)... And reverse the hood on the lens for storage, since it also nicely protects the focus ring from bumps. I never use mine without the hood, which might help explain why it's had a long, long happy life and is still going strong.
(Note: The EF 50mm f/1.8 II is a cheaper, more plasticky lens with slow/noisy micro motor AF... now discontinued. Cheaply built, they've been known to break in half! The EF 50mm f/1.8 STM is an improvement with faster, quieter AF and seemingly a bit more durable... though still compact and relatively inexpensive.)
Frankly, if I were you and looking to use the lens on full frame like yours, I'd opt for an 85mm f/1.8 or 85mm f/1.4 instead. I'm just not a big fan of 50mm on full frame... But I've used it a lot as a short tele/portrait lens on crop cameras. This is just a personal preference... I know a lot of people really like 50mm on FF. I'm only mentioning it because I didn't use it a lot when I first got it, as I was only shooting film at the time. I got such a good deal on it, that I couldn't pass it up and was glad I bought it because soon after I started shooting digital crop-sensor cameras. I love it as a short telephoto for portraits with those cameras, especially, and ever since I've used it quite a bit more.