Justin's reasons for selling his 24 match up well with why I never got one.
Most of the time that I'm looking for 24mm, I'm almost instinctively reaching for the TS-E 24. And I'm not looking for speed at those times.
I think most people would be very happy with either the 24 f/1.4 or the TS-E 24, but I don't think very many would get a lot of use out of both. They're both 24mm L lenses, yes, but they're not at all interchangeable and have no overlap except focal length. One might think that therefore there'd be room in the kit for both...but, as I see it, if you're happy with the one the other isn't going to interest you much except as a very expensive novelty.
And I couldn't be more thrilled with the TS-E 24....
I think the 24L II's utility depends on whether one shoots primes or zooms. I see it as competition with a 24-70 or a 24-105 (zoom vs. prime).
In this I disagree. I have a multi zoom/prime kit, and the only zoom that I've seen produce a "look" like the 24mm prime is the new 24-70 ƒ/2.8 L II, wide open. It would, I think, complement someone with a 24-105, since that lens isn't particularly sharp at 24mm and starts at f/4... you're missing a lot of bokeh at that aperture. One of the reasons behind my original purchase of the 24 .14 was that I thought I could use it's shallow depth of field to isolate subjects in busy environments that I couldn't control.
My only fault there was that I didn't account for my own inability to properly focus the damn thing.