Choice is always good, but the price is a bit silly, Sigma aren't Leica.
I like Sigma's prices, they are dirt cheap, which is funny why they're asking what amounts to 1/4th the lens price.
True... Then again switching systems just to continue using third party lenses is kinda... odd.
No one switches systems for the camera, the 5D2 used to be the ultimate IQ camera, now it's the D800, next generation of cameras it might the the other way around again; but your lenses are always the same.
I feel like you haven't seen their recent lens announcements...literally every lens they've announced since they're global vision has been a top lens...the 35mm blew everyone away...their new 18-35mm f1.8 lens is ridiculous to be that fast of a lens and be a zoom...and it's super sharp...they've yet to introduce a dud of a lens with this new global vision...

