Years of development have brought us a lens that has a fast aperture of 1.8 - far faster than any consumer zoom lens - and that is sharp as a filed tack. Be forewarned about the sharpness . . . if you are taking pictures of people, this lens is unyielding in its sharpness and may well surprise you and your subjects whose every blemish is captured. The lens has a fabulously shallow depth of field if you want to use the 1.8 aperture to blow out a background. This lens is also ridiculously inexpensive. It is not USM - so it is a little loud. It does not have a moving focus scale. For the money though - this is heaven.
As to the build quality - yes, it is plastic. No, it's not built like the Rock of Gibraltar. If you are going to give this lens extensive use as your everyday lens and you shoot a lot, it may not hold up all that well as one reviewer suggests. However, I've now had this lens and used it fairly regularly (although not as the primary lens) for about 8 years and it is still in great condition. In my mind, spend the $$ on this first before you go and drop $330 on the 50mm 1.4 USM lens and I think you'll find it gets the job done nicely and that the extra $250 on the 1.4 may not be worth the difference in build (major difference), speed (minor difference) and image quality (minor difference).