How would you compare the 24 against the new Sigma 35 for portraiture?
I'm currently using the Canon 50 f/1.4 on a APS-C body, and while it's great, it is a little too long for indoor work, unless you are doing tight crops.
So, if you wanted a wider lens to complement your 50 (x1.6 = 80) for portrait work, would you go with the Canon 24L (x1.6 = 38.4) or the Sigma 35 (x1.6 = 56)?
If you're sticking with APS-C you might also want to consider Sigma's 30mm f/1.4, it's a solid performer, maybe not "pro" quality, but I owned it for a while and got really good use of it. Or maybe even the 40m f/2.8 (it's sharp and low cost). The Sigma 35 f/1.4 is pretty remarkable, look for my review very soon.
24mm isn't particularly flattering for portraiture, environmental work, like weddings of photojournalism, where you want to isolate a subject within an "area" are definitely great uses for this lens, and yes, also very good on a crop body, even avoiding a lot of the lens "flaws" like vignetting.