1. Short and light--great for traveling.
2. f/1.8 for fast, sharp shooting.
3. Wide open aperture lets in lots of light so shutter speeds are very fast
in Aperture Priority mode.
4. Cheap--just over $100.
5. Great in low light without flash.
Warning: If you want a 50 mm prime lens that is more durable and a bit faster, go with the Canon EF 50mm f1.4 USM Standard & Medium Telephoto Lens for Canon SLR Cameras. It will last a lot longer than the f/1.8.
The advice above follows a you-get-what-you-pay-for approach. You'll spend a couple of hundred dollars more on the f/4 lens. Don't fret, though. If you go to sell it, you can get just about all of your money back if you sell it used on ebay or other similar website.