Let me start off with a disclaimer, I am not in any way endorsing Canon or any of its products. This is just my personal opinion based on my personal experience with their product, particularly the humble 50mm 1.8II lens.
The Canon 50mm 1.8II is not made up of any expensive material, doesn't have the bells and whistles that all the other lens have. As a matter of fact, once you hold it, you kinda get the feeling that it is not made for the working pro. It is made of plastic and feels flimsy.
But this lens is on my camera body 90% of the time. From products, portraits, photo journalism, and on some occassion, even interiors, I have used this lens and produced outstanding images with it. It focuses fairly fast (it doesn't have USM so let's be forgiving), it's tack sharp (as far as my clients and me are concerned), and my favorite part about it is it's very light ( I handhold a lot!).
More than the technical aspects, what I love about this lens is what it does (and gives) to me as a photographer. I started my career as a photographer during the film days and have worked almost exclusively wiith the medium format. Back then, there is very little choice of lens and I have been forced to work with just the standard 90mm lens for my Mamiya RB67. Because of this, the discipline to carefully compose, plan and think about a frame before tripping the shutter has been deeply inculcated in me. To work with what is available, to crop out the unneccessary and to maximize the entire frame by choosing the composition well are the things that working with a prime lens has taught me. And in this digital age, I have likened the Canon 50mm 1.8II to my former favorite 90mm for my RB.
With the coming of sophisticated lenses, it is easy to get distracted and sometimes, even get lazy, because technology can assist you in every way possible. Technology is a wonderful thing but one should never forget that an image is created by a person and not by an equipment. A photographer (ideally) should be able to decide on how to frame a picture even before he looks through the viewfinder and should be able to create a good image regardless of how crude or sophisticated the equipment may be.
The 50mm that I have been using for more than 4 years has helped me maintain that (film) discipline and has helped me further hone my composition skill. This is the reason why I think that the Canon 50mm 1.8 II is the best lens.