First quick thought - yes, it is made of 'better' materials than my 50mm f1.8, but I wouldn't necessarily say that it's therefore better made. My 1.4 has a bit of wiggle and a little jiggle when shaken - something that the f1.8 has to a slightly lesser degree. I'm sure this is normal, however, and not the optics shaking around inside!
Focus speed on the F1.4 seems about as fast as the f1.8 too - certainly not noticeably faster. And focus motor noise is also on par for both lenses. So really, it's in the optics that the f1.4 needs to shine.
Does it shine there? Well, as yet I haven't made a direct comparison, but initial testing with the f1.4 confirms what I've been reading from others. At wide open (1.4) the lens is all-over soft. Not unusably soft - for portraits the softness is kinda nice - but it is soft nonetheless.
You can see from the above portrait of Emily and the 100% crop in on the eye that, although soft, the image still works as a soft portrait. The background is beautifully blurred, and the depth-of-field at f1.4 is just enough so that facial features remain reasonably sharp, but get increasingly blurred out from about the cheeks backwards.
Change the f-stop to f2.8, however, and things improve 100%. The focus point (on the eye) becomes stunningly sharp. Although as you can see, you do gain more depth of field. Lips and chin are now also in focus using my D300 cropped sensor body.
So from about f2 the 50mm f1.4 is super sharp. Again - not sure how comparable that is to the 50mm f1.8, but I'll make that comparison soon. In the meantime, I will use the f1.4 at a wedding this week and see how it performs. Might post a couple of images from the day as well.