As a former owner of the Canon 50mm/f1.8 lens, and a current owner of the Canon 50mm/f1.4 lens, I can say the most noticeable difference between the two is build quality. I was very pleased with the photos I took with my 50mm/f1.8. I didn't replace it until I dropped it (from about 4 feet onto soft ground), and it broke in two. I then decided to upgrade mainly as a safeguard against future breakage. The 50mm/f1.4 is clearly much more solidly built, but it costs about 2.5x the money. Either one works very well in low light (although the f1.4 obviously has a slight edge here).
If you're starting out, I wouldn't hesitate to buy the 50mm/f1.8 Canon lens.
Whether it's worth the extra price for the f1.4 lens probably has more to do with how gentle you are on your equipment than on the noticeable optical differences.
Update: The EF 50mm f/1.8 STM introduced in 2015 gives you most of the best of both the older EF 50mm f/1.8 II and the EF 50mm f/1.4. It's build quality is much better than the EF 50mm f/1.8 II, the optics are as good as the EF 50mm f/1.4 at f/1.8 and up, and it's as cheap as the EF 50mm f/1.8 II was.
