I would buy a 24-70/4 IS around 800-1000 USD if it would be sharp enough (we don't know yet how this new lens performs). 1500 is a little bit pricey for me. The new 24-70/2.8 is very sharp, but no IS and very expensive too.
I'm using an old 28-80/2.8-4 USM L now.
I had some Sigma lenses (70-200/2.8, 105/2.8 macro, 1.4x, 2x) but after a few years all had different problems so I turned to Canon lenses and never looked back.
I love IS that's why I'm using Canon system.