In concept, 24-105 with IS is such a useful combination. And since I didn't have the original to fall back on, I went ahead and ordered it--in spite of the fact that I was also underwhelmed by the early reviews. In the era of high-ISO sensors, it seemed like a logical update for my classic 24-70 f2.8. Unfortunately, it's release was delayed beyond a deadline i had to meet. Since a top-end fast 50 was the other lens I wanted to complete my arsenal, I cancelled the 24-105 and ordered the Sigma Art 50. It is heavy for a 50, but then the new 24-105 has put on a fair bit of weight compared to the original. (Crotchety Ken R has decreed the new version too plump to be a suitable walk-around lens.)
And as advertised, the Sigma is a very nice lens. With a little legwork it can be made to cover much of the sweet spot of the 24-105. I put it on my 5D4 and handed it to my niece, who is a professional wedding shooter (shoots a D700). She went nuts; ran around the house blazing away at obscure targets hidden in dark corners behind chairs, etc., and then marveling at the sharpness and isolation of the subject.
It would surely be nice to have IS, and the flexibility of the zoom, but at this point I--and my budget-- are thinking I could wait to see if Canon hits it out of the park with the third iteration of this lens. Once you've tasted the new 100-400mm II, you're inclined to hold Canon to a very high standard.