When laypeople talk about extremely wide-angle lenses, they will invariably use the word "fish-eye" in the same way that many people colloquially use the word "migraine" to describe any type of major headache. In both cases, the intent is commonly understood but the accuracy of the terminology is suspect. "Fish-eye" is properly used to describe a specific type of wide-angle lens in the same way that "migraine" is medically descriptive of a specific subset of headaches.
Ultra-wides are the go-to lens below the 16mm APS-C (24mm full frame) focal length. They are eminently more-practical than fish-eyes, if somewhat less dramatic. "Practical" is a good term to use to describe the Sigma 10-20mm f/4-5.6 EX DC HSM, which upholds the traditional virtue of costing less than the equivalent first-party equivalent. However, traditional Sigma lenses have also not met the same performance standards as those set by Canon and Nikon. To that end, does the benefit of better affordability hamper the quality of this lens?