I've compared the L and non-L Canon 100mm macro lenses and for me there wasn't any noticeable difference between them in IQ terms. Both are very sharp, both focus quickly too.
The L has effective IS which is probably its main advantage, but needs an adaptor to attach a Canon ring flash (though I don't have one anyway)
The non-L Canon EF 100mm f/2.8 USM Macro is an excellent lens and is ring flash ready. I would have bought one of these, but got a great deal on the L version so got that instead.
Others to consider, Canon EF-s 60mm Macro and EF 50mm f/2.5. Both very sharp and compact. The 60mm only works on crop bodies, the 50mm focusing is bit slow and without an adaptor only does half life-size images. The working distance from lens to subject is pretty short with these lenses, which might be an issue depending on what you want to shoot. Both have ring flash ready front fittings.