I wrote this for another thread -
The IQ and sharpness are very close (possibly a slight edge to the L). The IS is nice. I use the IS more for non-macro, but it is still useful for macro. So the L is a much nicer lens (IMO) than the non-Ls, but not necessarily a needed upgrade.
It was easier for me to justify. I had (have as I have not got rid of it yet) the original non-USM 100mm macro. So I got faster AF with the USM. The ability to use a tripod collar (that the original wouldn't do) was something I wanted (the collar was bought in less than a week of owning the L). I get weather resistance. I also had the money, so I could afford it (plus I got a good deal). I'm getting older, so IS helps more than I'd like to admit. IS is used more on non-macro shots as I like a tripod and/or flash for macro shooting.
If I had the USM non-L version (which takes a tripod collar), I might not have upgraded.
If someone doesn't have a macro lens and wants one and has the money, get the L. Otherwise either of the two non-Ls are still fantastic lenses.
I assume you have the USM-non-L. The only real benefits of the L are IS, weather resistance, slightly better in build, and slightly better IQ (which is debatable).