I have had the "Nifty Fifty" for about 1.5 years and that lens got me hooked on primes.
I find this 1.4 lens to be superior in most every way to the 50 1.8 II.
First, sharpness. It can be hard to nail the focus at f/1.4, especially on a 5D at closer distances. But when you hit it, this is still quite sharp wide open. Stopped down to 2.0, 2.8, wow. I shot a bunch of portraits last week between f/5 and f/8 and they were stunning.
As a point of reference, I have a 24-70L, 24-105L, 70-200 4L, and the 85 1.8 to compare this lens to.
I have a very hard time telling the difference in IQ between this lens and the 24-70 at 50mm. I usually have to look at EXIF data to figure out which lens I was using, unless the photo is so sharp you get a paper cut, then I can tell it was the 50 1.4.
Next--Price? After dropping $1200 US on the 24-70, and $2800 on the 5D kit with the 24-105L kit, $325 US just doesn't seem like a lot of money. This lens is relatively much more money than the 50 1.8, but I'll tell you, the 1.8 II doesn't see any use except on my Rebel XT in my kids' hands these days. If there were no 1.8 available for $90 I don't think anyone would think the 1.4 costs a lot. It doesn't, especially when you start looking at how much the L lineup goes for.
AF--If you are wondering whether to settle for the 1.8 MkII, I can tell you I enjoyed the II, but the 1.4 is far superior in AF accuracy, speed and volume. The 1.8 II is loud and buzzy, it hunts, especially in low light (and sometimes never achieves focus), and misfocuses quite a lot. The 1.4's USM is very nice. Not as fast as the L lenses I own, but worlds ahead of the MkII. It is quiet, and seems to be much more accurate and consistent.
Build--While not metal, and not L quality by any means, it is still far superior to the toy-like MkII. The plastic is tougher. I think it is the same build quality as the 85 1.8 (seems like it). Also, there is a real focus ring here which I find to be quite smooth and useful. It has a metal mount too, not plastic like the MkII.
Bokeh--The MkII bokeh is chunky and blocky. The 1.4's bokeh is much smoother and is very much more pleasing to my eye. While not quite up to the bokeh I get from the 24-70L, it appears every bit as good as the 85 1.8.
In your kit--The 50mm f/1.4makes a great complement to the 85mm f/1.8 as I have both a 5D and a 40D. With the 85 on the 5D and the 50 on the 40D, my wife and I can shoot a kids/family portrait shoot with almost identical focal lengths.
With the 50mm on the 5D and the 85mm on the 40D, it's like having a 50 and a 135mm and switching between the two bodies with those two focal lengths is very useful.
I have not noticed significant CA, as some have noted, but I did not go looking for it either, so maybe if I hunt I'll find some.
All in all, this lens is an incredible piece of glass for what I believe to be a very good price. Really, if you can get a good lens this good for less than $500, you can't complain can you?
I would highly recommend this to anyone, even if you already have the 1.8 II. While the Nifty Fifty is an awesome bang for the buck and the two lenses seem to be pretty close IQ wise, the 1.4 is still a huge step up in quality.