Yes they work, I'm sometimes using one with my 100mm non-L macro with good results. However, there is a caveat: Canon and Sigma teleconverters do not work because they physically do not fit. There is a protruding bit at the front of the converter that fits into a corresponding cavity on the back of the lens - and only certain L Canon lenses have this. This means the various 70-200s and the longer tele primes, as far as I know. I don't know if the 100 L macro has this opening, from photos I've seen of it it does not look like it.
I am using a Kenko brand 1.4x teleconverter thas does not have this protrusion, which makes it fit just about any EF lens you'd care to mount it on. I put it on a 17-40L once, for laughs. They do make a 2x converter too. Basically the same products are sold under the Kenko, Tamron and Soligor brand names.