This is the cheapest of canon’s macro lenses. While not up to L glass standards I have no particular complaints about its build quality or optical quality. Since I don’t spend my time taking pictures of test charts I can’t really comment on its absolute optical performance beyond saying its adequate and I hear that its slightly edges out the nifty 50 in optical quality (but then given the extra cost it probably should). One technical issue is that instead of the 1:1 magnification most macro lenses give the the 50mm Compact Macro only offers 0.5 magnification. The focus system is rather noisy and the extending barrel can be annoying in some situations. The manual focus ring looks like it should be a weak point but it seemed to stand up to a fair bit fiddling.
So how useful is the lens for wikipedians? For proper macro photography (full lighting setup and effectively a tabletop studio) its probably going to be worth spending the extra money to get a lens that allows proper 1:1 magnification. For more casual out and about use extension tubes are cheaper and lighter. Still those involve a lot of lens changes unless you are prepared to put up with losing infinite focus. The 50mm Compact Macro allows you to shoot at post macro distances something which is quite common if the commons category is anything to go by. On a full frame 50mm also tends to be a bit short for the kind of macro work you might do in a museum. Small items are often far enough back from the glass that something a bit longer comes in handy. The situation with crop sensored cameras is better (75mm equivalent) however I don’t own a crop sensored camera. WikimediaUK does though so I’ve lobbed the lens in their direction if anyone want so try it. Eh if all else fails at f/2.5 it is faster than your standard kit lens.