Canon and Nikon produce the best wide-angles without a doubt, but you pay for the privilege of OEM build quality and IQ.
But the Siggy is popular, although it's worth noting that Sigma does produce the odd 'Friday lens' that will have to be recalibrated - you hear much on here about bad Sigma lenses that have to be adjusted, although that's probably down to selling more than other 3rd-party competitors. The Siggy is wide too, but that glass has to work herd to give sharpness across the whole frame and for me, that was where the Sigma couldn't match the Nikon the Canon or the Tokina. It is a budget lens of sorts (the variable aperture version being cheaper than the f/3.5 version) and doesn't claim to be anything else, but it's not perfect....
Personally, I'd go for a Tokina; the 12-24mm f/4 isn't a pretty lens and the version I had featured the wierd push/pull focus mode ring, but optically it's brilliant. Build is good, nearly on par with the Nikon I had, and it was sharp across the frame without any massively noticeable corner distortion, the distortion where it looks like someone's smeared the image in each corner.
By all accounts the Tokina 11-16 f/2.8 and the 10-17 are supposed to be very, very good also.
The Tamron is probably the black sheep of the bunch, never really echoing the success of the other tamron lenses that are amazing (28-75/17-50).
And that 2mm focal length difference is nothing more than a step backwards. Not great if you have a sheer drop behind you but in 99.9 per cent of situations it won't matter....
Have a butchers at
www.photozone.de - very reliable lens reviews (with example images) that should give you a better idea of what each lens does....
