The IS version is a newer design, it was released in 2006 whereas the non IS version dates back to 1999! Optically it is better, it has rounded aperture blades and overall build quality is better too. I also think it looks better, especially the switches. The IS version is weather-sealed too.
The IS is rated to 4-stops, and I have managed that and more. You can get shots that would require a tripod or really high ISOs otherwise. (One downside is that the IS motor is NOISY!)
If you shoot with a tripod all the time then all that is almost irrelevant and the non IS version will suffice. For my style of shooting IS makes all the difference.
I often wonder about the 70-300L, and whether I should have bought that. It all depends on individual shooting prefrences. Though, if I need reach I quite simply just plonk it on my 7D and am happy.