I'm expecting to be able to get my hands on one fairly soon (within a month maybe), though a side by side comparison with the old version may be difficult since I don't have one on hand to do that test with right now.
I never put a lot of faith in the Canon MTF plots. I believe they are theoretical and don't take diffraction into account, plus they don't really tell you what the image looks
like in the regions of lower MTF. Lower MTF can be a result of coma, astigmatism, field curvature and chromatic aberration and though they may all lower MTF, they all look different
My gut feeling, based on experience, is that unless you specifically want the STM feature for video work, I'm not sure it will be worth the cost of "upgrading", which currently looks like it will be around $250
The "upgrade" money might be better put towards another lens, like maybe a Canon 35/2
(while they are still available) or maybe a Canon 50/1.4
It's hard to see Canon making significant quality upgrades to their most basic and inexpensive kit lens while keeping the price "rock bottom" and it's still going to be a pretty slow zoom even if it's a bit sharper.
Video sharpness probably isn't a good guide because video is heavily downsampled and the sharpness depends as much on the downsampling software as it does on the intrinsic lens sharpness. However the silent operation and focus speed control of the STM focusing motor is a plus for video.
Time will tell though. When I test it it will be an unbiased test. If it is that
much better, it should be obvious from my MTF and resolution tests as well as my visual assessment of the image.