It's an ef lens. Will work on full frame or crop sensors. I use mine on both my 20d and rebel xt (both a 1.6x crop). Former owner used it on his 5d (full frame sensor). ef-s lenses will only work on your crop sensor bodies, the ef will work on both.
I would like an L lens in this focal range to compliment my 70-200Lf2.8, but, IQ is there on the sigma so I can't justify shelling out more money. That sigma lens pretty much stays on my body. I sell alot of prints, mainly landscapes, and even on a crop sensor, at 24mm, it's wide enough for alot of situations. If I need wider, I've got 12-24 atx. But what's great, after a day of shooting outdoors, I can walk in the club and not change a lens. I too shoot alot in the magic hour of the day, alot of it is of area landmarks where I need the low light capabilities without a flash and the f2.8 really comes in handy. I also shoot kids, and fast glass is a plus there. It's just easier if I have faster glass, I don't have to change lenses quite as often or haul around as much glass. 3 lenses in my bag is heavy enough.
Don't get me wrong, i still lust after more glass. It's like mics, I want more, but what I have is suitable for all situations...
I do so want a 5d though