I've had the 50mm f1.8 for about three months now, so I wanted to put in my two cents worth after a little field use. What originally attracted me to this lens was, obviously, the price. It is very, very inexpensive. This is likely due to the fact that the housing is, unlike its predecessor the Mark I, entirely plastic. That initially put me off, but after seeing some images posted that had been taken with this lens (and after seeing the prices of the f1.4 and the used mark I)I decided that I really had nothing to lose. There are, as with most lenses good and bad elements to this lens. Lets start with the bad.
Keep in mind that if you are shooting a canon DSLR (as I am) this 50mm lens actually behaves as an 80mm lens, so it isn't that terribly wide. The fact that it is functionally 80mm can make framing shots a bit difficult. This is definitely a secondary lens and really isn't that appropriate for a "walking around lens." At least it isn't for me, as I tend to prefer shooting wider angles.
If you have some sort of mishap with your camera, like dropping it, you can likely kiss this lens goodbye. I have fortunately never had to test this, but I imagine that it wouldn't stand up to any sort of impact very well. The flimsiness of the build is very obvious when compared to some of the older canon lenses. MY 35-135mm USM is about 10 years old, and has a metal chassis. These lenses can often stand drops and still operate. This is not so for the 50mm mark II.
Since everything but the glass is bare bones, the autofocus isn't terribly fast. If all you have ever worked with is USM lenses, you will have to be ready to take a little more time focusing. If you have experience with the 18-55mm kit lens, you will find that it is about the same.
All that being said, you are probably wondering why I rated this lens at four stars instead of, say, three. That's because there are a lot of nice features to this lens that far outweigh the bad.
If you have never used a prime lens before (meaning, a "fixed" lens that doesn't zoom) then you are in for a pleasant surprise. It is far more expensive to build a quality zoom than a quality prime, thus decent zooms tend to cost a mint. Also, zooms are only at their best in the middle of their range. The 50mm doesn't move, and so has been optimally designed for its focal length. Shots are sharp at all aperture ranges; shots at medium apertures (f8-f11) will blow your mind. Really. Search the web for some images made with this lens in those ranges and you will buy it. Resistance is futile. Canon may have skimped on the body, but they didn't cut corners on the glass. It is excellent.
The wide aperture (f1.8) is really outstanding as well. I had never really worked with a lens this fast before because, frankly, I hadn't been able to afford to. You dispense with your flash and handhold at levels that you would not have thought possible. And once you get away from flash use during night/lowlight photography, you will see some truly amazing colors that flash typically obliterates.
The fact that it is fixed, and not too wide, forced me to be a little more creative than I normally would when framing shots. When I went to Burma this year, I left my principal lens at my hotel, and didn't realize it until I was far away, so I was forced to use the 50mm as my principal lens. Some of my favorite shots of my whole trip actually come from that day, as I was forced to come to terms with the focal length limitations of the lens.
And finally, there is the price. A lens with glass this good that costs less than a filter setup, or a dozen 8x10 prints? How can you say no? Unless you have the bucks to spend on the f1.4 or the Mark I, this lens belongs in every EOS shooters bag.