Should I consider buying the EFS 24mm STM when I have a 28mm f2.8 classic?

I have both a full frame and an APS-C and the original 28mm f2.8 lens. I've heard a lot of great things about the 24mm f2.8 STM but is it worth getting if I already have a 28mm? I like to use both my full frame and APS-C camera.

Thanks guys!

Not really if you want to use it on FF. The 24mm doesn't work on FF. If you really need the added 4mm on the wide end on crop, then it's worth it. Otherwise, it isn't. I have a 24mm STM and I think it's great for the price but it's not special enough to switch from something similar unless you need the pancake size.

...but neither does it make 'sense' to keep the old 28/2.8. Trade up to the new 28IS.

The 24mm STM is a good lens - sharp in the centre, but perhaps not so much so in the corners. It also doesn't have IS, so there's another strike against it. And while there are those who would say that you don't need stabilisation on a 24mm lens, I think it can be useful on an APS-C body which is where this lens has to be mounted - it's an EF-S lens - and the effective focal length is around 38mm.

The lens' saving graces are two-fold. First,it's tiny! ridiculously small and light!! On a smaller crop body - 100D/200D, or a Rebel/XXXD - you don't notice it's there. It's a really good walk-around lens on these bodies. Perhaps a bit long, maybe, but do-able. Secondly, it's very cheap.

As you can see from the shots below I've used it when hiking and when just walking around, especially in hot places where the light weight is valuable. Of course, only you can decide whether or not it's worth buying.

Here are a few shots I've taken with it. Different bodies and dates but the same lens.

Covered market, Chania. 100D + 24 STM

Samaria Gorge, Crete. 500D + 24 STM

Barrow Stones, upper Derwent Valley Derbyshire. 750D + 24mm STM

I would suggest ef 24mm is usm f/2.8.  The ef-s 24mms is light awesome, but no IS, not as high quality, and won't attach to full frame.