Originally Posted by Lead-Foot
couldn't you just do fish eye with software?
You can, and you can't. It really depends on the shot you are getting.
A proper fisheye will be your only option if you are trying to get something that needs to be captured in a single 1/1000th of a second exposure. (Think guy on motorcycle hitting a jump and capturing it from the near foreground) For very static shots you can get away with faking it in post from lots of photos stitched together, but there is always the risk that you botched something, or the lighting changes, and it is generally just harder to work with.
If you have the spare cash, I say buy a decent quality one with good reviews for corner sharpness (Honestly I would argue that your corners on the ultra wide angle are almost more important than your center) and play with it for awhile. If you find you don't actually use it much, then you can always sell it.
Sadly I'm headed in the other direction, and saving money for good fast telephoto glass. Which makes the wallet cry...