If cranking up the iso was a non issue in photography, then we wouldn't spend the money on fast lenses. Take this lens into the forest where everything is bathed in shadow and tell me that having a lowest aperture of 6.3 at the long end isn't limiting... take this lens to the racetrack on a rainy day... take this lens to an indoor equestrian show or a hockey game... take it to the opera or any live stage show... take this lens anywhere where lighting is not ideal and it's limitations will become extremely evident.... especially when attempting to shoot at 600mm and you have to increase your shutter speed.
No doubt this lens offers great value for what it is, and it's great to see that it's sharper than expected and that it renders out of focus areas very nicely, but the fact is, the long end of this lens is what is actually appealing to people, and it would be more interesting to see manufacturers putting their R&D energy into figuring out how to give photographers affordable fast lenses... and in this case, taking away a portion of zoom range could be an approach that might begin to make that possible.
Maybe the good stabilization performance of this lens will make it more useful than I suspect... but I suspect that most of the people who purchase this lens will have buyers remorse before long. I know that for my style and my needs, I give it a pass...