Currently I don't have a wide-angle lens other my Tokina AF 10-17mm f/3.5-4.5 AT-X DX Fisheye zoom lens. So I thought I'd see what it's like at each focal length when defished (on my Nikon D200 DSLR).
For defishing the fisheye photos, I used PTGUI. I know DXO has defishing built in, and there is also the PTLens plugin for photoshop or editor for Lightroom. I processed the NEFs straight through PTGUI, so the resultant photos have a bit of Chromatic Abberation. If I'd processed the NEFs through Capture-NX first, this would have removed the CA, but I didn't bother since I was only interested in looking at the rectilinear distortion for this test.
Tokina AF 10-17mm f/3.5-4.5 Fisheye lens photos defished - focal length comparison
Here are the (mostly uncropped) defished images overlaid on top of each other. I'm not sure why, but the 14mm and 15mm images both take up about the same amount of space. (Click the thumbnail to view a larger version).
And this is what the images look like when they've been cropped down:
I would say that the Tokina fisheye zoom is just about usable as a rectilinear wide-angle lens from 13mm upwards. There is also another projection method that could be used instead of defishing to rectilinear - Panini / Vedutismo projection. I didn't know about this projection method when I did the tests, but it looks like it could work well with the fisheye, even at 10mm: The Vedutismo / Panini Projection.
Another projection method I have tried is Equirectangular projection. When using this method, a photo will defish okay without distortion at the left and right edges, but you will get nasty bends if you don't place the horizon line around the center of the image. And if you didn't place the horizon line around the center at the time of capture, you will find that placing the horizon center when defishing means a lot of the image will have to be cropped off.
Fisheye lens vs. wide angle lens
So, do I think a fisheye is a suitable replacement for a normal rectilinear wide-angle lens? In short, no. There are a few of problems with using a fisheye and then defishing it:
Unless you're using DXO or some other software to process your photos that can automatically defish the photo when converting the RAW file, it takes a bit of extra work to defish the photo. Not a problem if you're only doing the odd photo, but if you're using the fisheye as a replacement for a wide-angle lens, it could get quite tedious and take up quite a bit of time.
Defishing the photo obviously involves distorting it, and so the resultant image will very likely not be as sharp as one taken with a regular wide-angle lens.
With a normal wide-angle lens, what you see through the viewfinder is what you get in the final photo. If you're using a fisheye, and then defishing it, what you see and what you get can be quite different (especially at 10mm, not so much at 17mm). This makes it difficult to frame your image, as it is hard to be sure how much will get cut off when you defish the photo .
But if you don't have a normal wide-angle lens, then a fisheye lens will do in a pinch. And of course, the fisheye lens works very well for fisheye images, 360° panoramas, and is wider than a rectilinear lens of the same focal length.