Concerns about 100mm Macro USM - can it be sharp without IS or expensive equipment?

I compared the two 100mm macro lenses earlier this year and ended up springing for the L version and spending the extra $400.  I would have preferred the non-L but after trying both for 10-20 minutes I decided the L was well worth the extra $400.  Some of the things I noticed were:

1. The L was faster focusing

2. The L has slightly better contrast and color rendition.  These are fairly slight but definitely noticeable.

3. The L's IS really makes a difference.  I was able to get a number of low light or small aperture shots that came out blurry with the non-L version.  This means I can get handheld shots with the L that would require a tripod with the non-L which makes the L version much more versitile.

4. The L's build quality is definitely better than the non-L.  Probably means it will stand up to rugged use better.

5. Not sure if it is the better contrast that does this but sometimes the L appeared slightly sharper than the non-L.

From research I did before buying a 70-200L f4 IS - in general the L lenses hold their value better than the non-L lenses.