I wouldn't want to be restricted to full aperture on something like a 28-20mm but it's no great problem on the 400mm, which I often use at either F5.6 or just one stop down - the very close-focus of the lens does benefit from a bit more stopping-down for depth of field purposes, though.
I've seen comments before about the D30 and D60 being OK, which is quite interesting as both of these models came after the EOS 30 which seems to be where Canon first got around Sigma's chips.
It's a competitive world out there and price is everything these days, but one wonders just how much extra each lens would cost if only Sigma had paid Canon a licence fee for using their technology - I suppose they thought that they were being very 'clever' getting around Canon's patents without realising that Canon would do the same to them!
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