One of the best situations in which to consider a third party lens is when the camera body manufacturer does not have an equivalent offering. Meet the Sigma 20mm f/1.8 EX DG Lens - as of this writing, Canon does not offer a lens this wide with an aperture this large. The Sigma 20mm is over 1 stop faster than the fastest Canon 20mm prime or zoom lenses (f/2.8).
Unfortunately, I consider the Sigma 20 to be unusable at f/1.8 unless you are looking for a soft-focus effect. This lens is one of the softest I've seen wide open. Realistically, this will be an f/2.8 lens to most people as the center does not become decently sharp until this aperture. But wait - there goes the Sigma's uniqueness advantage for now there is a direct Canon counterpart, the Canon EF 20mm f/2.8 USM Lens that costs about the same price (price at times is another reason to buy third party lenses).
At f/2.8, the Sigma is sharper in the center than the Canon 20mm, but the Canon easily wins the corner competition even on a 1.6x FOVCF body at all apertures. My Sigma is worse on the right side than the left (the ISO 12233 chart mid and corner crops are from the right side), but the difference is not enough to merit much thought. Sigma's own MTF chart even indicates miserable performance outside the center. The corner problem seems to lie in the Sigma having a significantly curved plane of sharp focus - which I suppose no longer qualifies as a "plane". I am able to get reasonably sharp corners when the Sigma 20mm lens is stopped down, but the sharp areas are not close to the primary plane of sharp focus. At f/4, the Sigma 20mm f/1.8 EX DG Lens is sharp in the center and even sharper in the center at f/5.6. Below f/8, the corners are terrible.
Moving on - Vignetting. The first line in the first paragraph of Sigma's 20mm lens web page claims "This super-wide-angle lens is ensured minimal light-fall-off with superior peripheral brightness." I don't know what they are looking at, but the Sigma 20 turned in some of the strongest vignetting I've seen - 5 stops in the full frame corners at f/1.8. The not-so-funny thing is, the Canon 20mm Lens turned in similar results at f/2.8 and remains over a full f-stop behind the Sigma in vignetting performance as it is stopped down. Full frame users will want to stop this lens down to at least f/4 or f/5.6 to get more even results. Users of 1.6x FOVCF bodies will likely not notice much vignetting.
The Sigma 20mm f/1.8 EX DG Lens is very flare-prone - more so than the Canon 20 and much more so than the Canon EF 16-35mm f/2.8 L USM Lens. Keep the sun behind the large 82mm Sigma objective lens.
If you can get past the softness, vignetting and flare, the Sigma 20mm f/1.8 EX DG Lens actually produces decent image quality. Colors and contrast are good. Foreground/background blur quality are nice (9-blade aperture). Some CA (chromatic aberration) is visible at the wider apertures. Full frame body users will see mild barrel distortion. Exposures tend to average at least .25 stops brighter than typical Canon lenses.
The Sigma 20 internally focuses accurately and clearly lets you know that it is accomplishing this. The loud autofocus motor is not terribly slow, but the sound makes it seem much slower than it really is. Manual focus utilizes Sigma's not-so-brilliant Dual-Focus mechanism. The advantage to DF is that the focus ring does not turn during autofocus, but manual focus requires a switch change and a pull-back on the focus ring. The problem is that I am frequently changing the focus ring setting by accident when simply handling the camera/lens combination. Autofocus still works, but the focus rings turns in your hand - risking damage to the lens. Canon's USM (on Canon's 20mm lens) and Sigma's HSM are much nicer MF implementations. The manual focus ring is nicely sized - and smooth.
The Sigma 20mm f/1.8 EX DG Lens is nicely sized and solidly built. At 3.5 x 3.5" (88.6mm x 89.5mm)(diameter x length) and 18.3 oz (520g), this lens is a joy to handle. The wide 82mm objective end is the reason for wide diameter measurement as most of the lens diameter is less than 3" (76mm). The large, abrupt change in lens diameter is not a nice comfort feature, but the small lens hood (included) mounts at this point anyway. The lens finish is the standard Sigma matte finish - I'm not fond of this, others like it a lot.
82mm filters are not cheap, but the non-rotating front element is a positive feature. Split Neutral Density and Circular Polarizer filter users will appreciate this. The Sigma 20 is probably best-suited for wide landscape photography where these filters (and narrow apertures) are commonly used.
I can think of a lot of other uses for a 20mm f/1.8 lens, but most of them require a sharp image at wide apertures. This would be a very-handholdable, action-stopping lens if it could be used at f/1.8. Of course, if you don't mind soft images, the look of a closeup 20mm f/1.8 picture is very artistic looking. With a short minimum focus distance of 7.9" (20cm), the Sigma 20mm lens yields a nice .25x maximum magnification. These specs coupled with the angle of view provided by a 20mm lens and the background blur of an f/1.8 aperture create very attractive images.
The Sigma 20mm f/1.8 EX DG Lens is available in Canon (reviewed), Nikon, Sigma, Minolta and Pentax mounts. My standard disclaimer: You should be aware that there are some potential issues with third party lenses. Since Sigma reverse engineers (vs. licenses) manufacturer AF routines, there is always the possibility that a new body might not support an older third party lens. There are examples of this happening in the past. Sometimes a lens can be rechipped to be made compatible, sometimes not. Second, there is the risk of a problem that results in the lens and body manufacturers pointing blame at each other. However, Sigma USA's 4-year warranty is far superior to Canon's standard 1 year warranty.
In conclusion, I recommend not buying the Sigma 20mm f/1.8 EX DG Lens. Sorry Sigma, this one is a dud. If you want a fast 20mm lens, I suggest saving for the Canon EF 16-35mm f/2.8 L USM Lens. It far surpasses the performance of the current Sigma and Canon 20mm lenses in all regards. If you want a good 20mm lens but do not need a fast aperture, the Canon EF 17-40mm f/4 L USM Lens is a great choice.