(Lots of words and numbers to follow, scroll to the bottom for the tl;dr)
The Canon 50mm 1.4 is the first lens that I've got for my camera that isn't an old film camera lens or the kit one. Canon make a bunch of 50mm prime lenses ranging from under £100 to over £1000, this one sits in the middle of the current range at around £240 new. The main differences between the range of 50mm lenses Canon makes is the build quality and maximum aperture, the more you spend the better you get.
The field of view that a 50mm lens gives you on a film or full frame camera is very useful and makes a good general purpose lens, this is why there are so many old ones about. I have a few old 50mm lenses which I love which is one of the reasons I wanted this one. On a crop sensor body like the Canon 600D that I use the field of view is much tighter, giving it an equivalent view of an 80mm lens. This makes it more suited for portraits and gives much more emphasis on the subject. It also means that on all of Canon's sub-£1000 cameras it doesn't make a great general purpose lens and you almost always need to take another one with you. The 50mm focal length is also one that is included in the standard 18-55mm kit lens so there is really only one reason to buy this lens - that aperture.
This lens opens all the way to f/1.4 which when compared to the kit lens at 50mm, lets in 16x more light! So when shooting at parties or in the evening you can keep the shutter speed higher or ISO lower, resulting in less noisey images. It also means that you can get much shallower depth of field, again great for portrait shots. When used wide open at f/1.4 results are good but there can be some darkness in the corners and softness in the image, it's not that noticeable and stop down to f/2 or f/2.8 and it's lovely and sharp across the frame.
The build of this lens is worlds above the kit lens and especially the 50mm 1.8. It's got a solid metal mount and feels great in the hand thanks to all that glass inside. The focus ring is really smooth and you can turn it to focus manually at any time, even in auto-focus. This is really handy when you want something specific in focus but your camera is having other ideas. It is also really nice to use in manual focus when filming. The only complaint I have is that as the lens extends when focusing it becomes vulnerable to knocks which can damage or break the focusing system completely - not fun. I keep a lens hood on mine at all times which the lens extends inside of to protect against this.
The following video was shot on the Canon 600D and 50mm f/1.4
I'm glad I added this lens to my collection and love the results so far, can't wait to see what I can capture in future!
TL;DR Great lens, but a bit zoomed in on a crop sensor and may not be worth it for most over the cheaper 50mm 1.8.