I have a 24mm F/1.4 II L but I recently bought a 28 F/2.8 IS lens because I thought it would be a good, light weight contender given it's great reviews.
FAIL - What an ordinary (or less than) disappointment that was.
I think my 40mm F/2.8 gives better IQ.
One of the worst lenses I've ever mounted on my 5D MK3...
The 24/1.4 II is a phenomenal lens — one of Canon's best — but much bigger, heavier and costlier than the 28/2.8 IS, so you can't expect the 28 to be as good. I use both lenses for their strengths. I've found the 28/2.8 IS to be a good lightweight & compact alternative, and often a more useful focal length than the 24. The only fault I've found with the 28 is the rather strong vignetting wide open, but that's been a minor issue (easy to fix, or leave as is when it looks good). And the 28/2.8 IS and 24/2.8 IS are both more useful for video than the 24/1.4 II. I think the point of these lenses is that they have different strengths; otherwise there would be no reason to make them.
And, for what it's worth, LensRentals tested the Canon 28/2.8 IS wide open at f/2.8, http://www.lensrentals.com/blog/2012/06/the-other-canon-primes-why-did-they-do-that
, and got these results:
... which happen to be the exact
same resolution results as the awesome new Sigma 35/1.4 stopped down to f/2: