Are there any downsides to buying a third party extension tubes for Canon?

Question

I would like to get a little higher magnification out of my Canon 100mm Macro, so I am looking at extension tubes.

I know Canon makes the EF25, but it seems pretty expensive considering there isn't even glass involved. I saw a "Kenko" extension tube set that is only a little more expensive for a 12, 20, and 36mm. Are there any downsides to buying a third party extension tube?

Answer

As far as I know there should be no real difference. The extension tube have no lens elements but is merely (as the name suggests) a tube. The Kenko extension tubes feature the same electrical connections as the Canon one (judging from reviews and images on the net). According to this review, there are two versions of the Kenko tubes; an older one that is not compatible with EF-S lenses, and a newer one that is. Worth keeping in mind if you have such lenses.

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