I do more macro than anything else, but I have never used the 180, so take this with a grain of salt.
First, it matters what body you are shooting with. If you are shooting at minimum working distance, the body does not matter, but if you are not, you will have more reach (hence less need for a longer lens) on a crop body. I shoot macro with a crop body, so take that into account.
I do a LOT of flowers, and I would not consider a 180 for those. The extra working distance is not particularly useful with flowers, and the difficulty in handholding a 180mm--particularly with a crop body--would be a real problem for me. I shoot some flowers with a 100L, but I actually do more with an EF-S 60mm macro. For some flower work, I find the shorter working distance helpful.
Bugs are a whole different issue. I shoot bugs a good bit as well, and sometimes for that, I would like a longer focal length. However, 100 does fine.
IS is not that usefull for macro work
Yes and no. Conventional IS compensates for angular motion and is pretty much useless when you are near minimum working distance. However, the hybrid IS on the 100L compensates also for motion parallel to the sensor, which matters a great deal for macro work. I find it buys me about 1.5 stops at minimum working distance. Also, if you want to use the lens as a telephoto as well, the IS on the 100 works as conventional IS as well, giving you 4 stops at distance.