I don't have any L-glass optics so can't compare to those.
I do have the Sigma 70-300mm F4-5.6 DG APO Macro and the Canon EF 70-300mm F4-5.6 IS USM, and the difference between those two is remarkable.
Before I decided on the IS lens, I google'd and found this page:
Test Shots
Shows pretty well the differences.