Gordon Laing at Camera Labs writes:

Canon EF 24-105mm STM review so far

The unique selling point of the EF 24-105mm STM is its focusing system: it becomes (and so far remains) the first full-frame EF zoom with Canon’s lead screw-type stepping motor, allowing it to focus quietly and smoothly in movies and live view. It’s also the only full-frame STM lens with image stabilisation and that’s wider than 40mm. These all make it the ideal walk-around companion for full-frame bodies with Dual Pixel CMOS AF, and in my tests it punches above its weight in terms of image quality. I miss the weather-sealing of the L models, but the optical quality for the relatively low price makes it a bargain in the Canon catalogue, and if you want an STM zoom for a full-frame body, it’s the only game in town at the time of writing.