I'm not nearly as sold on this lens. Everything about it seems GREAT except that most people get the 18-55 STM or 18-135 STM with their camera and at 23mm those lenses shoot at f/3.5. That's only 2/3 of a stop difference but you get IS with them. This is the same conclusion I came to after trying out the 24mm IS lens, and then selling it. I think if you're looking to spend $150 on an f/2.8 pancake prime, it's smarter to go with the 40mm as it provides a significant advantage over the kit lens lens which is at f/5 as soon as 37mm making the 40 1&2/3 stops faster.
Now, that's not to say no one should buy the 24 IS, just that it shouldn't be a blanket recommendation for everyone.
Disagree, though I understand your point. You don't buy/not buy a lens solely for max aperture reasons. That's just one consideration.
- Both 24 primes you referenced will be sharper than the kit zooms, esp. on the more wide open end, say f/2.8 - f/4 or so.
- The pancake is as big as a minute with only vignetting, focus by wire and STM focusing speed as it's only limitations. Just like the EF 40mm, it's shockingly sharp for its size.
- The 24 f/2.8 IS is in another league entirely. Sharpness equivalent to the 24L II. It's a fantastic lens with modern fast USM, internal focusing, great build quality and of course the IS. That's effectively a "slow" 24L prime without weather sealing, and I honestly rate it as good a value at $549 as the pancake is at $149.
Kit lenses are great, but both of the 24 primes you referenced are clear upgrades over kit glass to me.