I have the 10-17mm fisheye and can say that it's a very fun lens. If you don't mind some distortion, especially and close to mid-range, you are rewarded with a very wide view. From my initial playing I can provide some advice, keep your fingers back from the lens unless you want a crop the bottom of your photo anyway.
Here are some example shots. I'm still practicing and some of the shots were a bit of a challenge due to the high contrast range, but these should give you a rough idea of how the fisheye will perform for landscape work. I'm still mucking around with them and might also try some HDR methods to tweak out some more range.