A few months ago I rented a Panasonic 7-14mm ultra-wide zoom, and had a great time using it. Since I'm using a micro 4/3 camera, it's the equivalent of 14-28mm on a full-frame camera. Unfortunately, it's very pricey and holds its value well, so the number of times I would use it doesn't justify the expense. However, I noticed that there are some very inexpensive ultra-wide adapters, and thought it would be worth the $10 to experiment with one. The adapter is pretty small but has a surprising heft to it. Since the element construction is very simple, the focal point doesn't extend very far back (ultra wide angle lenses use a reverse telephoto construction to extend the focal point back to the sensor). To make the lens useful, mounted just behind the front element is a macro adapter, making the image usable by the lens to which it is attached.
Indeed, the lens does greatly expand the field of view.
Even at this size it's very clear that the center is sharp enough, but the edges are not sharp at all. It seems that the top and bottom are close enough to the center that they didn't lose nearly as much as the left and right. I'll need to play with it a bit more to determine how much of that was the focus being off, since it looks like the focus wasn't quite at infinity.
These photos don't show it as much, but there's quite a bit of barrel distortion with the lens. Also, there's strong vignetting in the corners (cropping to 3:2 eliminates that).
In the end, it's not great, but is certainly worth $10. I think I'll still be able to get use out of it, especially knowing its limitations.
For those who are interested, the photo of the adapter at the top was taken in relatively low light with a phone. With post-processing, I was surprised that it came out as well as it did.