An extra f-stop is always an extra f-stop, with all that comes with it:

• Narrower depth of field (perhaps marginal in comparison to the f/1,8)
• Brighter viewfinder; great in low-light situations
• Might be the difference between getting the shot or not, also in low-light situations
• Lenses tend to get sharper when stopped down. At f/1,8 the f/1,4 lens is stopped down one step, while the f/1,8 is wide open. I don't know if there is any actual difference in sharpness between the lenses here, but I would assume that the f/1,4 is sharper at f/1,8 than the f/1,8 lens.

Is it worth the price difference? Only you can tell...

I had the same question, when i bought the 85mm 1.8, instead of the 1.4. I am happy w/the results & money that i saved. The 70-200mm 2.8 costs over 2k, & is much slowerthan 1.4, but 2.8, still gives a nice bokeh. Camera companies, know how to juice the market, by offering 1.4, but no a 1 stop difference ....makes no difference.

I love my 50mm lens and I use it all the time. In fact I have two of them; an older 50 f/1.8 and a brand new Canon 50mm f/1.4. I really love the 1.4 version because it focuses extremely quickly and it’s incredibly sharp. The 1.8 is also very good (especially for the price), and I happily used it for years before upgrading, but it focuses really slowly, especially in low light. In my experience the major differences are:

• the 1.4 focuses faster, and is very quiet
• the 1.4 is sharper and has much nicer bokeh (the soft, out of focus highlights)
• the 1.8 is much cheaper (approximately 25% of the price of the 1.4)
• the 1.8 is cheaply made, but also much lighter

So if you can't afford the 1.4 then the 1.8 is a great lens. But if you can afford it the 1.4 has many advantages over the 1.8 and I would definitely recommend the more expensive lens.

I have a 50mm f/1.7 (slightly wider than the one you're looking at) and it's a great lens, without a doubt but, the extra stop on the f/1.4 doubles the light that hits the sensor when wide open and that can make a real difference in certain circumstances. However, if you're not avidly trying to take shots in low light without a flash, then the extra stop may not mean all that much. So, my suggestion is to base your decision on the intended purpose of the lens for your photography.

I think it depends a lot on the circumstances. An f/4 to f/2.8 jump seems a worthwhile jump (neglecting price factors) if you need the low light performance or shallower depth of field (eg for sport). A jump from 1.8 to 1.4 really is at such a shallow depth that it's not often useful as often. I find even for low-light concert photography 50mm f/1.4 is a little too shallow at times.

As John mentioned though, there are other factors in specific lenses (AF performance) that make them more attractive.

Light light light.

Also, you can get some really beautiful close up portraits of people with even more razor sharp depth of field. I'd definitely go for the 1.4, a better lens is everything, and for a prime, you getting the best value for money in terms of glass quality