The EF 50mm f/1.4 is a nice lens, it is more often than not the one that is mounted at any given point on my camera. The f/1.4 is a bit faster than the f/1.8, a bit sharper, and has a bit better color, but it doesn't completely blow the f/1.8 out of the water. In my opinion, its greatest advantages over the f/1.8 is it has a metal mount, and 8 blade aperture (f/1.8 has a five blade, so the f/1.4's bokeh is a bit smother), and most importantly, it has full time manual focus (you can manual focus without having to flip the switch off of AF, this is something I can not live without).
The downside of the 50 f/1.4 is the AF motor is that it has the micro ultrasonic motor, not the ring ultrasonic which is in all of the nicer lenses. This makes the focus ring a bit stiffer, less of the silky action you get on an L lens, and the micro USM has a higher failure rate then ring USM (manual focus just stops working). The barrel also extends a small touch when focusing (I think the f/1.8 does that as well), and it doesn't come with a lens hood. All that said, I do love that lens. Would I rather have the EF 50mm f/1.2 to get the ring USM (and a slight bit faster), absolutely, but the f/1.4 is a pretty good lens for the money.
Since you already have the f/1.8, is there another focal length that you are interested in, or is the f/1.8 just not cutting it for you?