Well, I haven't owned this lens in a couple years, but after someone favorited a photo taken with it on Flickr, it reminded me that I never wrote a review before I sold it. If you do some reading on the internet, you'll see many different opinions about this lens. The majority of the reviews complain about this lens being soft, but I'm here to hopefully prove that wrong.
Fairly small lens, pairs very well with smaller crop sensor cameras. Notice the small rectangular shield just behind the front element to reduce flair and ghosting.
First off, let's talk about size and build quality. It's small, light, and fairly solid. Metal mount with sturdy plastics for the construction. It's small size and weight makes it wonderful for street photography. It fits nicely in most jacket pockets, unlike the Canon 35mm f/1.4L or 24mm f/1.4L. Most of the time I would pair this with a 50mm lens, allowing for quick swapping of lenses on the street. Although it is not weather-sealed, I've used this lens many times in light rain without any issues.
Canon 5D, Canon EF 28mm f/1.8 @ f/2.8. Out shooting on a cold and snowy day. No problems at all.
While the lens doesn't come with a lens hood out of the box, it does have a rectangular plate behind the front element to reduce flair and ghosting. How much it actually does, I have no idea, but I never had too much problem with lens flair. A lens hood is available from Canon for extra money. I bought one, but most of the time, I never used it as it just made the lens bigger.
Shooting directly into the sun, that's the most flair I could ever get out of the lens.
So let's talk about the sharpness issue that some people bring up. Yes, it is not as sharp wide open as it is stopped down. That's also true of most lenses. From about f/2.8 and smaller, sharpness is even across the frame with good detail. At larger apertures, it does take a small hit in sharpness on the far outside corners, but it really isn't too bad. The center wide open sill remains sharp with great detail. Wide open at f/1.8 is more than usable and looks great. Having that flexibility and extra light can be critical for low light photographs.
Canon 5D, Canon EF 28mm f/1.8 USM, f/1.8. Beautifully sharp and very nice depth of field fall off.
Stopped down, things of course get nice and sharp. Very little vignetting and beautiful color rendition.
Canon 5D, Canon EF 28mm f/1.8 @ f/5. Nice even exposure across the frame. little vignetting present.
Fast apertures on wide angle lenses. If you aren't shooting low light, why would it matter? Environmental portraits with subject isolation. Getting shallow depth of field on wide angle lenses can be difficult, especially since most are f/2.8 or smaller. There's some vignetting wide open, but I've always been a fan of it and the way it can focus the viewer's attention.
Canon 5D, Canon 28mm f/1.8 @ f/1.8. Beautiful subject isolation and bokeh. Can't get that field of view with a 50mm.
Canon 5D, Canon EF 28mm f/1.8 @ 1.8. You can really notice the vignetting, but the in focus area is sharp! Beautiful bokeh as well.
Let's talk about auto focus. It's actually pretty good. I never shot sports with it as normally a wide angle lens isn't super useful for it, however It was always responsive for street photography and live music. Snappy, quiet, and accurate thanks to it's ultrasonic motor. Like all USM motors, it allows for full-time manual focus override.
Canon 5D, Canon EF 28mm f/1.8 @ f/3.5. The infamous Don't Call Me Betty of Bloomington, IN
There are some the alternatives as I mentioned earlier. Canon 35mm f/1.4L or Canon 24mm f/1.4L. Sigma also makes a 24mm and 35mm, both of which are f/1.4. Of course, these lenses are larger and much more expensive. If size or money is an issue, than the 28mm is a better option at half to a quarter to a third of the price of a fast 24mm or 35mm from either Sigma or Canon. The Canon 28mm f/1.8 can be found on the refurbished market for about $400, and new for closer to $500. A fast 24mm or 35mm can range from $900-1500. There is also the Canon 35mm f/2 of which there are 2 versions, the first with terrible auto-focus, and the second for a lot more money, but with image stabilization.
Canon 5D, Canon EF 28mm f/1.8 @ f/7.1. Ah. The snow and winter in Bloomington.
Now we come to the end, or at least closer to it. I used the 28mm as my main wide angle lens for a couple years, then I bought the Canon EF 24mm f/1.4L USM II. I simply needed a faster lens. Of course the 24mm is wider, and for some situations was too wide. So I bought the Canon EF 35mm f/1.4L USM, which became my new favorite wide angle lens. So then I was "stuck" with 3 fast wide angle lenses, 24mm, 28mm, 35mm. I realized that out of those lenses, I wasn't using the 28mm anymore as my shooting style changed. I preferred the speed benefits of the 24L and 35L, and the majority of the things I photographed, size and wieght didn't matter. Shortly before moving to Dallas from Bloomington, IN, I sold the 28mm. (hence why all the photos are from Bloomington).
In summary, the Canon 28mm f/1.8 USM is a great lens, but if you've got the money and aren't set on 28mm focal length, the Canon and Sigma versions of 24mm f1.4 and 35mm f/1.4 are better. If you need the low light advantage, that 2/3rds of a stop can make the difference.