As a long-time user of the Canon 50mm f/1.8 lens, I was always envious of photographers that owned the f/1.4 version. And then I got one.
I would never go so far as to say that this lens is bad in any way... it is, in fact, quite good. But its image quality isn't quite as high as I expected it to be, and it doesn't have anywhere near the bang-for-the-buck of the f/1.8 version. Image quality is only marginally better than the f/1.8, and while it uses a USM motor (not ring-type) to focus, it really doesn't focus that much faster and isn't terribly quiet while doing so.
Build quality is better on the f/1.4, but only marginally so. It has a metal instead of plastic mount, but the body of the lens is still plastic. Being able to manually focus at any time is nice, but not something I would consider essential. Having a USM focusing system instead of the micro motor in the f/1.8 does give a feeling of higher quality, if focusing manually is your thing.
So I actually have a really hard time recommending this lens to someone who already has nifty fifty. It's only marginally better, yet costs between three and four times as much.
Image quality at f/1.4 is a little soft across the entire frame, but it is noticeably worse in the corners. It doesn't really sharpen up fully until f/2.8, but we don't buy this lens to shoot it at f/2.8 -- we buy it so we can shoot in really low light or to get maximum bokeh at f/1.4. If you don't mind some softness, f/1.4 does meet those requirements, but don't expect pictures to be tack sharp when doing so.
For photographers looking at lens options, I'd have to recommend getting the 50mm f/1.8, and then the 85mm f/1.8 instead of the 50mm f/1.4. You'd be getting two great lenses for only a little bit more than you'd spend on just this one. The 85mm f/1.8 lens is significantly better than this one in nearly every way, and it is roughly the same price. And 85mm is much better for portrait work (especially on a full-frame body), if that is your intent. If you're looking for a lens for portrait work, just skip right over both 50mm options and go right to something at 85mm or longer. 50mm is a poor choice for portraits.
Bottom line, if you're shooting portraits, just get the 85mm instead. For anyone else, unless you need full time manual focus, you've got money to burn, or you're really set on getting the f/1.4 for its slightly better build quality, I'd recommend getting the 50mm f/1.8 instead. It's 90% of what this lens is at roughly a third the price. Then invest what you would have spent on this one into the 85mm f/1.8. But as far as the difference between the f/1.8 lens and this lens, there really isn't that much of a difference. This lens is very good, but so is the f/1.8.