Thank you for all the replies so far!
My main use for the macro will be food close ups to start with, but I will also use it for a variety of other purposes (e.g. I travel a lot and often find myself wishing for a macro lens while doing it). As I mentioned I was hoping to use the lens as a portrait lens as well (f/2.0 would sometimes be useful here), but on the other hand I wouldn't want to compromise its primary use.
I could be wrong, but I suspect that by "food close-ups" you're not really implying the sort of "true" macro photography that others have been addressing - do you want, say, to have a solitary beautifully lit caviar egg fill the image? If you merely want to "get close" but let the image be recognizably of food, all you may need is a lens with a short minimal focus distance or a lens with sufficient magnification. Macro lenses are great for that too, of course (that's primarily why I like them so much), but for such purposes it will matter less whether it's 1:1 or 1:2 or even whether it's 50mm or 100mm or 180mm (by all accounts the Sigma 180mm IS lens is fantastic, but if the food you want to photograph is in a restaurant chances are you won't want to us *that* lens...).
The 100L is one of my favorite all-purpose lenses, for all the reasons already given by others. The IS won't always be helpful, but sometimes it will and I would rather have it than not; and while AF isn't as accurate as MF when operating with shallow depth of focus, especially if you care about - and you will - which precise details are in focus, it's nice to have it otherwise, especially when you're using it on a dslr, hand-held, where MF is rather hard to do effectively. But if you don't need IS and don't need 100mm you could by the 70mm Sigma, which does 1:1 magnification, has superb image quality, costs half as much as the 100L and makes an excellent all-purpose walk-around lens. Or you could try the 100mm non-L Canon. Or, frankly, just about any macro lens - as far as I can tell, they all provide impressive image quality, with differences among them being relatively trivial in actual use.
(If you had a decent mirrorless body it would be easy to recommend some cheap macro lenses to start (or even end) with. E.g. for less than $150 you can buy a Nikon 55mm f2.8 MF lens + adapter and get superlative image quality; it also has the advantage of being small and light (a mere 289g) - but I wouldn't want to try to use mine on a dslr....)