I have had the 35m, 28, and 20mm. The 35mm was useless. It was too close to the 50mm lens. The 28mm is used very often, especially in Europe. The 20mm rounds out the wide angle and gets a fair amount of use. I have not tried anything below 20mm, which for me is too close to being a fisheye.
As far as fish-eye lenses, they would not be used enough for me to purchase. Besides, I do not like taking photographs with my feet in the photo!