I've currently got the 100mm non-L and I've shot literally tens of thousand of pics with it. When I researched the initial purchase of my gear I read that there was no difference in sharpness between the two and the IS wasn't really beneficial for 99% of my shooting, so I saved some money and went with the non-L, probably an ok decision at the time.
But now that I've shot a lot, advanced my gear collection a lot and read up more on L vs non-L I've decided that I will upgrade. Apart from IS there are a few other differences. The L is a tad sharper actually (see Photozone for example). The weather sealing is welcome, plus there is a focus-limiter switch and the L bokeh is better as well. Not sure how the Tamron compares, but the 100L seems good in many different areas.
I'd like to know if there is an AF advantage to using the L but I suspect that the body is a more important factor.
Interestingly I had the internal motor of my 100mm lens conk out one time, the rig just siezed up and said I needed to clean the contacts, but actually it was a motor problem. Cost me AU$260 to fix, but considering how much I've used the lens I wasn't really annoyed. But it has made me rather paranoid, so on serious macro holidays in future I will be taking along two lenses (100L/non-L and EF-S 60).