Upload & Sell: Off
I've recently acquired the Sigma 24-105, but having moved house within the last week, I've not had much opportunity to use it properly.
My initial impressions though are that it's plenty sharp enough even wide open, colour rendition is so close to my Nikon primes it's not worthy of further mention, AF is fast and very quiet. The strangest thing is the OS, it's so silent, I thought mine was broken at first. Sharp hand helds at 1/30 at 105mm on a D800 proved otherwise, though.
I considered the Nikon 24-120, but opted for the Sigma instead.
Optically, I've yet to see a proper comparison against the 24-120 on a D800 or a D7100, but I expect it's going to be at least as good. The Canon version is at least as good as the 24-105 f/4 L, so for now we'll call it a draw.
Price-wise it's cheaper than the Nikon, but it takes 82mm filters and a decent CPL will take care of the saving, so let's call that a draw.
Build quality is where the differences really start to show. The Sigma is big, heavy, solid and superbly engineered. It feels like it's built to take a lifetime of professional abuse. The Nikon, on the other hand feels like one of the throw-away kit lenses that come with consumer DX bodies. Here, in the UK, the Sigma carries a three-year warranty vs one year for Nikon. Value for money, the Sigma wins hands down.
The huge difference many overlook with the new Sigma lenses is the USB dock. Naysayers like to shout that it's Sigma's way of compensating for sloppy quality control. I call BS on that. Sigma has come up with a proper solution to compensate for the ever increasing demands high resolution sensors are placing on phase detect AF systems. Nikon bodies have offered AF fine tune for a few years, but it's a single adjustment. I've yet to find a zoom that needs the same adjustment across its full range, and most lenses have variations with focus distance. Sigma's dock allows tuning at no less than four focal lengths on zooms at no less than four focus distances, letting you dial in a lens near perfectly for all conditions. For me, this is a winner.
Firmware can also be updated to keep lenses working with future camera bodies, something Sigma has been criticised for in the past (although to be fair they will usually re-chip current lenses for free, if needed).
Sigma has entered a new era - they're concentrating on quality and performance, and their recent offerings show they can make lenses at least as good as and in some cases better than Nikon and Canon at very competitive price points.
The die-hard fan boys will still benefit, because it will force Nikon and Canon to innovate and compete to maintain sales.