One relatively common question we get is, "Should I get the Canon EF 100mm f/2.8L Macro IS USM or EF 100mm f/2.8 Macro USM lens? Is the 100L worth the extra cost?" And those are certainly good questions. Of course there are comparable third party options to consider, but for the purposes of this post we'll be looking at the two 100mm Canon models typically considered.
Both lenses feature the same focal length, the same maximum aperture, offer 1:1 macro focusing capability and feature very similar image quality. While the 100L barely edges out the non-L in sharpness at wider apertures and the non-L is better with flare, I wouldn't consider image quality to be a differentiating factor between the two lenses. However, there are a couple of key differences between the lenses that aid in differentiation.
Probably the biggest advantage of the EF 100mm f/2.8L Macro IS USM is its Hybrid Image Stabilization. Hybrid IS corrects for both angular and shift movements and allows for roughly 2-stops of correction at 1:1 focusing distances and up to 4-stops of correction at longer focusing distances. If you plan on using your macro lens handheld in the field, the L-series lens – with its HIS, great build quality and weather sealing – will be your best choice. The value of image stabilization for handheld macros cannot be understated; it's hugely beneficial. And considering that the 100L is only about 50% more than its non-L counterpart (in the US, without rebates), many photographers will find the pro-grade lens's benefits worth the investment. Being one of Canon's least expensive L-series lenses means that this lens is often a photographer's first introduction into Canon's premium lens lineup.
For those who prefer using a tripod when photographing macro subjects, and who do not need weather-sealing, the EF 100mm f/2.8 Macro USM provides L-series image quality at a more wallet-friendly price. I [Sean] personally opted for purchasing the EF 100mm f/2.8 Macro USM for my own personal use and I have rarely regretted it. However, I rarely shoot macros handheld. Instead, I typically prefer to capture macros under very controlled circumstances employing a solid support system [tripod] and strobes (whether indoors or outdoors). Under those studio or studio-like conditions, the benefits of the 100L are mitigated if not entirely moot.
In summary, if you plan on shooting macros handheld and/or need weather sealing, get the EF 100mm f/2.8L Macro IS USM. Otherwise, save some money and enjoy similar IQ with the EF 100mm f/2.8 Macro USM.