It’s reasonably fast with a max aperture of f/2.8, and reasonably wide-angle with an equivalent field of few of 38mm on a full-frame sensor. This makes the Canon 24mm f/2.8 STM on an APS-C body a close cousin to the Canon 40mm f/2.8 STM in terms of field of few and speed on a full-frame body, such as the Canon 6D. And indeed, the two lenses share similar technology (stepper motor for focusing) and physical appearance. The minimal size and weight also makes it an ideal candidate for travel photography. And with an inexpensive adapter, it can be mounted on any of Canon’s mirrorless M-series camera bodies, such as the EOS M.
The max aperture of f/2.8 is pretty fast, especially for a $149 prime lens. It’s very compact at 2.68 x 2.68 x 0.91 inches and weighs a mere ~4oz (~118 grams).
It is respectably sharp wide open, with some minimal edge softening and vignetting. Focusing is fast and consistent, but it’s stepper focusing motor does make more noise than Canon’s USM focusing technology found in other Canon lenses.
The STM focusing technology in this lens allows for continuous AF operation (focus tracking, etc) during live view and while shooting video. This will be a big plus to videographers who have missed this feature in past Canon lenses.
The bokeh, in my opinion, is nice but not great. It’s smooth enough in many instances, but can get a little harsh and noticeable for mid-depth-of-field details. But, for a wide-angle lens, this is not something I consider an important flaw.
The STM focusing system does not allow for full-time manual focus. You must activate manual focus with the switch on the lens barrel. Additionally, the camera must be powered on for manual focusing to work, because even in MF mode the stepper motor is used to set focus.
I don’t think there is much bad about this lens. For the price (even brand new), it delivers very good image quality, pretty good bokeh, fast and reliable autofocus, and it’s small and light. Hard to complain too much with all that going on.
More details and additional sample images are here: https://tightcamera.com/canon-24mm-review/