I have the Canon mount version of this lens and it has a new street price of about $500.00. I labored long and hard for a wide zoom that would make my 1.6X Canon 20D be wide enough for indoor photography. I wanted something that wasn't too expensive yet very sharp. This lens is SHARP. I almost bought a used 20-35 2.8L but was put off by some of the negative comments and mixed reviews. After reading the reviews on this lens and the fact that it had a higher rating at photodo I decided to take a chance on it. The fact that it was almost $200.00 cheaper new than the used Canon was a nice plus. I own two Canon L series lenses a Canon 100mm macro and a Sigma (the Sigma is disappointing) and can compare this lens with some of the best. After the Sigma I thought I would never buy another third party lens. I am very happy with this lens.
The build quality is very good and feels solid in your hands. It has a metal mount though most of the lens is a well finished plastic. A little lighter weight than I am used to, but that is comming in handy. Focus and zoom functions are smooth.
As with all wide angle lenses the filter size is large and will cost extra dollars for them. This is because it requires a large filter to prevent vignetting caused by a filter. However, I still recommend a thin filter for this lens.
On a DSLR it is only barely wide enough but it does the job. If you want truly w-i-d-e this isn't the lens for you. It is very sharp and even the edges are good by f4. It is quick focusing but noisy and is not prone to hunting under normal lighting. Also, it is fast with 2.8 being the maximum aperture out to about 28mm retaining its speed through most of the focal length. There is some barrel distortion at 20mm but clears up by 28mm. Since most lenses at this length are prone to distortion this isn't much of a problem (and this lens is better than most). Some have reported some distortion at 40mm but I don't see any.
Vignetting is slight but present at 20mm as well but is gone by f5.6. At 40mm it was not noticeable. I also took some "test" shots of the sky and dark edges (for a worst case scenario) and chromatic aberration was virtually non-existent at any focal length. The APO elements of the lens seem to do their job.
There is a slight tendency towards the warm side but not objectionable, actually it reminds me of the old Kodachrome 25 slide film (one of the best all time films) in the way it renders color. If you don't like it though just change the white balance. Just as an aside, most probably wouldn't notice this unless two pics of the same scene with different lenses are used.
Lastly, flare... well it is bad. You will have to avoid shooting into light sources and use the included hood.
Two things keep me from giving this lens a 5, flare and build quality. While built very well I just can't give it a 5 as there is a lot of plastic.
Is this lens worth the price... YOU BET!