I sold a perfectly good 100mm Macro to buy the 100mm L (partly because I got a great deal) but it was a dissapointment, it's not really any better optically and the H-IS doesn't make enough of a difference to justify the price.
My advice to anyone wanting a lens for macro would be to buy the cheaper 100mm and a macro flash, that will give better images than even the very best macro without a flash. (Cue dissent!)
There are a few niggles with the 100mm L too, they don't get mentioned much in all the adulation - it's not a perfect lens, far from it.
The lens hood is ridiculously large so much so that it's unusable for macro photography, if it doesn't scare away your subject it will block out so much light that you won't be able to focus accurately!
The body is plastic so are the lens filter threads it's easy to strip them!
The lens is larger than other macro lenses so the macro flashes won't fit and you have to buy an adaptor like a step down ring - simple enough buy Canon will relieve you of Â£45 (Â£45 for a step down ring !!!!!) just so you can attach it. It fits the old one directly.
The H-IS is good for about 1 stop at 1:1 magnification I suppose every little helps, but it's just not enough to make a significant difference.
This isn't a cheap lens, it costs nearly 75% more than the non L version, and although it's a great performer, the old one was too, to be honest it's difficult to tell which images were taken with which lens.
Internal focussing makes both of these lenses very quick, especially when compared to third party offerings, but be aware that as they focus the focal length drops so the lenses are actually 67mm at macro magnification.