It depends what you want to do with it. I seldom photograph things that are moving around fast (and if I did I would probably use my 135L instead anyway), so the fact that the 100L isn't always very fast doesn't matter to me and the IS is very useful as light goes down when I'm not taking real macro photos (I seldom do that, either). A huge advantage to me of the L is its obviously shorter MFD - I may not want to take a lot of macro photos, but I often want to get close enough to take portraits of small things - flowers, kitten faces, etc. - and the three feet/one meter of the 100 f/2 often isn't close enough for me. Never having used the 100 f/2 I can't comment on its bokeh, but it's pretty wonderful on the 100L and, since you can get closer, you can get more blur in the background.
I had hoped to find comparisons on-line of 100 f/2 vs 100L with photos, but perhaps I didn't look long or carefully enough. But I did find this, which compares 85 f/1.8, 100 f/2 & 135 L which, if nothing else, might nudge you towards the 135L at some point (or not):http://www.dgrin.com/showthread.php?t=222419
As for the slow focusing of the L, well, it's all relative. Provided you stay within a narrowish range, it can be near-instantaneous, but if you switch from, say, a subject that's two feet away to one that's 20 feet away, it probably won't be and presumably (I've not tried) won't be fast enough for fast-moving sport where the distances keep changing significantly. The only time it's given me a problem, with considerable hunting, was in photographing some glass sculptures (hard to photograph in the best of circumstances) in a field late at night where the only source of light was the dim internal light in each sculpture. Manual override proved useful then (using the focus limiting switch would doubtless help too; I keep forgetting to use it). I've no idea of the f2 would have done any better.