If you own one EF lens, THIS should be it

The optical quality of this lens is amazing. It was one of the principal reasons I chose the Canon EF system. The USM function is beautifully quiet and INCREDIBLY fast on auto-focus. However, I usually use MF, which is the only drawback to this lens. The ring feels a little loose and cheap. But it works and works fine. If you need one really good lens to last through the ages, this is it. Plonk this on an older model Rebel and you will still get professional-quality pictures and tremendous room for enlargement if you do your own darkroom work. Of course, there is, as with all fast lenses, a fair amount of flare at 1.4. A lens hood should take care of the problem most of the time. I use that 1.4 setting a LOT (at night, in low light, when I want super-'bokeh', etc.) so it makes this far and away a better choice than the 50/1.8-II. Needless to say, it runs circles around the pathetic 28-80 that comes with a lot of EF bodies. The price difference between this and the EF 1.8 is pretty big ($295 vs. $79 for the gray market models.)* While the 1.8-II does not have the excellent USM function of the 1.4 and does not have a proper MF ring, it is a hell of a lot cheaper and the optical quality is almost as good. However, if you have a different set of needs and require a lot of focal lengths AND you're on a budget, you might consider getting the 50/1.8-II instead. Or, if you can bide your time, I would suggest getting this lens first and then waiting until you can afford other prime lenses. If you are on a VERY strict budget and simply cannot afford THIS lens, please pick up a 50/1.8-1 (not -II) used or on an auction site. They have a real MF ring and a metal mount and generally go for around $115.
*If you're going grey, you shouldn't have a problem. Do it at B&H and you will get a warranty that matches the US-edition one.