A couple years ago I rented an ultra-wide lens and had a great time seeing Seattle Center in a new way. I saw a used one go up for sale at a very good price, so I bit the bullet and got it, just in time for us to go to Vancouver. I ended using the lens well over 95% of the time. Some of the information in this post is similar to what Ken Rockwell has on his site, and that page gives a lot of great information about shooting with an ultra-wide. As you begin to use such a lens, some of his tips make sense as you see what's in the viewfinder.
The big difference with using ultra-wide isn't that you can get more into the frame (which is indeed true) but that you can greatly accentuate near from far, distort perspective, and use it as an improvised shift lens.
All the photos below were shot at 7mm (equivalent to a 14mm lens on a 35mm camera), except for the one in the Chinese garden which was 8mm.
Getting more into the frame
The use for an ultra-wide lens that probably first comes to mind is to get a lot of stuff into the frame. That actually ends up being harder than one thinks, because normally you want to get more width, but not more height. However, sometimes things work out so that you have interesting content vertically, too:
I didn't have much more room to step back without being in the street, so the wide coverage was much appreciated.
Another example of wanting to get more into the frame is this shot of the Sam Kee building:
What looks like a double-wide staircase is actually a single staircase against a mirror. The opening is probably not much more than 10 or 12 feet wide, and as you can see I was right on top of the near railing and was barely able to get everything in.
Accentuate near from far
Because you can get very close to an object with an ultra-wide, you can use that to your advantage to make objects look farther away than they really are. This is because the relative distances are greater. As an example, if the foreground object is five feet from the far object, and a regular wide-angle lens lets you get five feet away, the far object is twice as far as the near one. If you can get within one or two feet of the near object, the far one is 3.5 or 5 times as far as the near one.
Another way to think of it is an ultra-wide will exaggerate the size of the foreground object and the background. For instance, in this photo taken at Canada Place, I was probably not much more than a half foot away from the cable mount:
This photo shows you a more normal view of the area:
Another way I was using the lens was to better isolate something in the foreground. The ultra-wide allowed me to take advantage of the sky and water to make the post stand out in this photo at the Dr. Sun Yat-Sen Park:
With a lens that isn't as wide, the building in the background would have looked larger, making the post less prominent. Another thing you may notice that's different with this photo is the background is out of focus. The depth of field with this lens is so great, even wide open (only f/4.0) almost everything is in focus, so I have to get extremely close (a few inches) to be able to do this.
A very common use of ultra-wide lenses is to get close to the base of a subject and point up, as with the Canada Place cable. In addition, since you're pointing up, you can also get objects projecting into the frame from all sorts of angles:
This photo of the Sheraton (center) and Electra (left) buildings looks confusing at first, but I was standing right in front of the tree that's front and center, pointing up and towards the Sheraton. You can make out that there's a line of trees which is parallel to the face of the Electra, as well as trees which are further towards the lower right of the frame.
Another example of extreme distortion is this shot of the entrance to the T&T Supermarket:
You can imagine what the actual storefront looks like, but this angle accentuates the awning.
As common as pointing up is with ultra-wide lenses, I ended doing it a lot.
Improvised shift lens
In some photos I wanted to minimize perspective distortions but still get a lot into the frame. What I would do is shoot vertically but shift the object up or down in the frame, allowing me to hold the camera level. For example, here's a shot of the floor at the Chinatown Plaza Mall from a near-standing position:
The wall at the back shows a lot of distortion, so I stepped back a bit and took the shot again with the camera pointed straight ahead:
I then cropped the photo to landscape:
Since I stood further back and the cropping made the effective focal length a bit longer, the emblem on the floor is more compressed in the final shot than the original. The shot ends up being a bit less dramatic, but I find the final shot less disorienting. Even though I was kneeling for the vertical shot, I could have done so a bit more and kept the camera horizontal, but that would have still changed the shape of the design.
Here's another, more extreme example. This is the final shot:
and the original from which I made the crop:
I ended up standing a bit away to avoid having my shadow in the shot (which you can see in the uncropped version), otherwise I would have gotten closer so the crop would have been less severe. Even so, I ended up pointing the camera a bit down, which is why the pillars seem to be falling away from each other.
Since the final image came from the original image's upper right quadrant, you can imagine how much distortion I would have gotten if I used a lens that wasn't as wide but pointed up to get everything in the frame. I suppose another option would have been to get a lot farther away and use a longer lens; if you think of it as a right triangle, the tangent (rise over run) would have been smaller, so less distortion. If I could be a bit higher up (closer to the middle of the upright) it would have helped, too. A telephoto lens would have made front-to-back relationship more compressed, however.
While I feel comfortable pre-visualizing a shot with anything from regular wide-angle (say, 28mm equivalent) to telephoto, I'm still learning to see what an ultra-wide lens will capture. I'm at the point where I have a general idea, but I still find myself surprised when I look through the viewfinder.
Another thing I'm getting used to is being more aware of how level I'm holding the camera to keep vertical lines straight.
There were many times during our trip where I was thinking I should switch to a longer lens, but most of the time I didn't and tried to think of a way to make the shot work. I'm glad I did take a longer lens, however, since there were a couple times when it was the way to accomplish what I wanted to get out of the scene.
While I feel I still have a way to go, I'm not only getting more comfortable with an ultra-wide lens, I'll probably carry it instead of my kit zoom (in addition to a telephoto zoom and a fast normal prime), with everything still fitting in a relatively small bag.