Distortion and lens flare are the two bugaboos of wide angle lenses. When shooting landscapes it can be difficult to keep the sun out of frame. Some lenses, like the Canon EF-S 10-22 handle the sun well. Others exhibit flaring. And then there's distortion, which is dependent on the focal length. The wider the lens, the more distortion. My 10-22 is pretty good, but my 8mm FE will really show bendy horizons if it isn't held level. The distortion can contribute to the image as well, though.
