I have a 550d and I find myself using primes, 35L, 501.4, 60mm efs, smayang 85mm MF lens.
I'm missing something wider.
I really like quality primes and was saving up for a zeiss 21 2.8 or a canon 24L.
Now the new 24 2.8 USM showed up. My feeling is that it will be another "consumer lens" aimed a video primarily (the IS indicates this) and nothing special as far as IQ compared to the above.
Is it worth waiting for ?
Personally, I've found that Zeiss lenses use inferior multicoatings to recent Canon lenses, which results in worse...often MUCH worse...lens flare. If I had to pick any particular lens and my options were Zeiss or newer Canon, even if the size had a slightly better MTF, I'd pick the Canon. The significant improvement in multicoatings and greatly reduced flare would always be more valuable to me than better resolution but the possibility of intrusive, contrast-eating flare.
That aside, I'm not sure you would find a huge difference between the 35mm and a 24mm (its 36° vs. 50° FOV, or around 28% gain.) You would probably find something like the EF 8-15mm L Fisheye or EF 14mm L being far more valuable from an FOV standpoint... @14/15mm, you gain around 53% more FOV.