I've owned the non-L version of the lens many, many years... wonder lens. I've never thought of upgrading to L because I couldn't imagine a sharper lens, and to me, the IS hold little value as nearly all macro must really be done on a tripod...
I'm curious about the difference the rounded aperture blades of the L makes - I can't say I've ever been bothered with the non-L's effect, but it would be interested to see an A - B comparison between the lenses that highlights this difference.