That's not entirely true. 10 mm is perfectly usable out of the box, but if you are picky about distortion, you would want to correct it (and yes, I am, so that's what I do). Most people don't find it noticeable enough to bother.
But you should know that distortion happens with all wide angle lenses; it is a fact of life. And, the Sigma 10-20 is one of the best superwides in terms of distortion. It has better distortion characteristics (1.2% barrel) than the much more expensive Nikon 12-24 (1.7% barrel) and the Tokina 12-24 (2.3% wavy + barrel). The only superwide it doesn't beat in distortion is Sigma's own 12-24 (1.0%).
Keeping in mind that all of its competitors start at 12 mm instead of 10 mm (it's a lot harder to correct image problems at 10 mm than it is at 12 mm), the Sigma's optical performance at 10 mm is nothing short of phenomenal. I would even go so far as to say that I bet the Sigma 10-20 will perform
better at 14 mm than the Nikon 14-24 will, and will do a very good job of keeping up with the Nikon between 14-20 mm.
IMO, don't place unrealistic expectations on the 14-24. It's not going to be a flawless lens. It will have its flaws and shortcomings too, especially wide open and at the wide end. You can't change the laws of physics. And at such wide angles, f/2.8 isn't as valuable as it is at longer focal lengths. It sounds like you're putting it on a pedestal too much.
Well I did seriously consider the Sigma but the fact that I can't use
the 10mm range right out of the box without post processing bothers
me. And I have nothing against Sigma I have the 30 1.4 and love that
lens.
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dSLR: D40, D80, S5 Pro. Nikkor: 18-200/VR, 70-300/VR, 35/2. Sigma: 10-20, 50-150/2.8
P&S: A710is (+CHDK RAW mode), Fuji F10, S3is (sold)
