I have no used the non-IS version but I do own the IS L version. I looked at the other but decided to go with the L series mainly because I want to walk around and shoot macro pictures without the need for a tripod. The IS works very well and I have gotten some amazing flower pictures without a tripod. If you want really small subjects such as insects, you will still need a tripod. The L series weather sealing is nice too. I have to admit that the 100mm 2.8L is my favorite lens. It is amazingly sharp. I am using a 5DIII too. I say, if you can afford either, get the L.
This is handheld in a park with the 100 IS L with hybrid IS on. That was the first shot so don't worry about having to take a ton and hope one comes out not blurry. This IS rocks.