What keptwoman said is the same reason I also bought the 50mm (however I went for the 1.4, not 1.8. More expensive but I wanted the USM). I remember reading an article, (can't remember where, either laughingplace.com or Miceage). But the guy showed photos that he had taken on Small World. The same photo using the standard 18-55 and with the 50mm 1.4, and the difference was incredible. With the 50mm, he did not use a flash (obviously as they ask you not to), and the photos were amazing.
After we bought our lense, we took it to Disney on Ice at the Wollongong Entertainment Centre (sitting in literraly the last row too) and used it and most of our photos were great. The colour that I was able to capture and the image quality (in most cases) was amazing, so I would definitely consider buying the 50mm 1.8 if you can afford it. I will more than likely use it as my main lense on our next trip day or night and plan to get some awesome fireworks (if I can) shots as well.
The only downside is that being fixed length, you may have to consider some of the close shots or shots where you want a lot in the picture. For example when we do at home shots at parties etc, we need to go back to the 17-55 for the range, but that is only one down side I think there is.
One last thing, I have a link to a website (on the home computer) where a guy has a very good explanation as to why the Cannon 50mm should be in every one's kit bag as well as other lenses. When I get home, I'll attach the link to this post.