No, I haven't had the chance to compare the two Canon 100mm macro lenses. Since the old one was so good there can't be a huge amount of optical difference. The main practical difference is probably the hybrid IS system on the new lens, plus the new lens might be slightly better wide open, especially in the corners of the frame. Stopped down I doubt you'd see any significant difference.
It is true that IS is less effective at 1:1 macro than at normal focusing distances. I believe that Canon's specs are something like up to 4 stops at "normal" distances, 3 stops at 1/2 lifesize (1:2) and 2 stops at lifesize (1:1) This is basically because there's a lot more to stabilize at macro distances. At normal distances only angular movement of the lens causes an image shift, but at macro distances both angular rotations (pitch/yaw) and linear translation (up/down) shift the image and the higher the magnification the more linear translation moves the image. However based on user reports and published tests (which I have looked though), it appears to me that the IS does indeed provide maybe 4 stops at normal distances and up to 2 stops at 1:1, so I'd say that it's likely to be very useful.
Actually nothing is nearly as good as a tripod for 1:1 macro work because even if you stabilize up and down movement, you still have back and forth to deal with - and with razor thin DOF it's hard not to shift distance between focusing and shooting, and a movement of a mm or so may be enough to blur the image.
So if you only did macro work, you might be better off buying the non-IS lens along with a tripod and a
macro focusing rail. However if you intend to use the lens for things like landscapes, travel, portaits and other general work as well as for macro shots, then I think the IS system will be very useful indeed, and it will help on those occasions where you want to do macro shots and don't have the tripod and focusing rail with you.
As always it's a cost/benefit analysis. If cost isn't part of the equation then there is no doubt that the
EF 100/2.8L IS macro is clearly the lens to get. However if cost
is an issue, the
EF 100/2.8 Macro is still very, very good and about $430 cheaper. You'll just need to shoot at higher ISO settings or larger apertures in order to keep your shutter speed up to at least 1/125s in order to have a good chance of handheld shots. With the IS version you'd be able to go as low as 1/15s or even 1/8s at normal focus distances and maybe 1/30s at 1:1 macro.
You can certainly boost ISO to compete with IS, though if you are looking for 4 stops, that means going from a noise free ISO 100 to a pretty noisy ISO 1600.