I think these are a touch sell for Sigma...if I'm going to plunk down $5000 or more on a lens, I would want to know that it will always work on my Canon body....can Sigma guarantee that?
Guarantee? I'm not sure Canon can *GUARANTEE* that as technology mutates and matures over time, although presumably Canon will try harder. It looks like that is what Sigma is trying to accomplish with the USB lens dock that allows update of firmware and other tweaks. It remains to be seen how well that will work over the long haul, though.
I would probably be inclined to avoid Sigma lenses that were not compatible with the dock.
Also resale....at least a canon 300 2.8ii purchased for $6500 today will still probably be worth $5500 on the used market 5 or 10 years from now.
$5500 in 10 years sounds a little optimistic to me (unless it spends a lot more time on display in your china cabinet that out in the field on monopod), but yes, obviously the Canon lens will be worth more than a third party optic down the line.
But resale is such a foreign concept to me. Most lenses I have owned have been pretty much used to *DEATH.* My beloved 24-105 f/4.0L is the lens most showing it's age right now. I have to keep a constant grip on the zoom ring. If I have it pointed upward, it "falls" to 24mm, if it's pointed down, it "self-zooms" to 105mm. However, image quality is still great, and it gets used pretty much daily.
My only added concern with a Sigma lens might be that death could come a little sooner. I've never actually owned a Sigma lens, but I'm becoming more open to the idea all the time, especially as they release lenses nobody else makes like the 18-35 f/1.8 or 120-300 f/2.8.
If and when my 24-105 kroaks, that new Sigma 24-105 will get a look.