I fully agree, the OIS in the 50-140 (that I own) is much much better than the one on the 18-55 (that I used to own).
With the latter lens, I also found sharpness issues with OIS turned on and shutter speed around 1/125 - 1/250 so I often kept it turned off.
Some say it is obvious, you should always turn the OIS off with faster speed, but this is not the case with the 50-140.
So the lack of OIS is not an issue for me, and I would never go back to the 18-55. The heavier lens is sharper and micro-contrast is visibly better, to me at least.