I've been considering this lens for years. Or the non-L version. From everything I've read, it seems there are a lot of folks that prefer the non-L because it is as sharp or sharper and the non-L supposedly focuses faster.
Not directed to me, but my view.
I originally owned and used the Non L non IS version of this Lens, keeping in mind 95% of what I use this Lens for is Underwater Macro, when I went from the Non L/Non IS to the current lens, I was amazed at how much work Canon had put into this Lens. The difference in sharpness from the older version to this version I found remarkable in my own Imaging, the Focussing was faster, the IS certainly in my own use was a definite plus. All round I don't think there's any question the 100 f/2.8 L IS version is a few steps ahead of the older non L/non IS version, if your wanting to jump into one of these Lenses, my view would be to definitely go for the newer version.
I'm currently still shooting this Lens on my 5DMK II as I only recently set up a new Housing so I can in future shoot on the 5DMK III, I expect to see even more improvement then from the 5DMK III's much better Focussing system, and the fact that almost all my Canon Lenses respond better on the 1Dx & 5DMK III than they did on the 5DMK II, 1DMK IV & 1Ds MK III.