The 100L seems to be the best of the standard ~100mm macro lenses (with a price to match that).
There are cheaper options (e.g the tamron 90mm or sigma 105 (or even the non-L canon 100)) that will deliver excellent image quality but without IS and perhaps without the same mechanical quality (non-USM, maybe not as tightly constructed).
Basically, there aren't any true macro lenses on the market that don't deliver superb image quality, just pick the cheapest one with specs that suit your criteria (if you like handholding then a more expensive IS model would be good, if you want to shoot skittish insects a longer focal length can help (e.g the excellent sigma 150mm) or if you want it to double as a general purpose mid-telephoto then one with fast AF would be nicer to work with.