What do u mean by "crappiest lens" My copy is sharp wide-open in the centre at f2.5. I often shoot at f2.8 without any problems, despite the fact that my copy has some blurry right edge, which only shows up in shooting test charts. It is vy sharp wide open on the left edge and centre too on a full frame camera. It is a vy sharp lens good for food photography.
The only issue is QA like the defect I have and the loud focus motor noise LOL!
BTW, distortion wise, the compact macro tops 50/f1.8 & 50/1.4.
To begin with, I had to test 3 copies, before I could find one that was not badly centred (obviously yours is as well). Honestly, the edges are indecent at 2.5, and the complete coverage until the corners is not happening before f8. That's acceptable on the 50mm f1.8, not on a Macro lens. The vignetting is is also extreme until 5.6, and strangely it's the only lens I have that Reikan FoCal refuses to AFMA (seems the AF motor can not repeat focus properly enough). I have a 15mm fisheye that doesn't show the same issue.
The plus is the distortion is absolute 0, the field is perfectly flat (good for art repro), and it's a killer lens at f8, f11 and much better than most at f16. So I use it as kind of fixed aperture lens for architecture. Considering the price, it's not that bad, but again it compares very poorly with the Nikkor AI-S 55mm and AF-D 60mm I had before.
For a macro lens it is the absolute minimum requirement, and clearly shows it's age. I believe it's the only 1980's micro motor left in Canon's range (not sure if they still make the 135mm soft focus). I would buy immediately an improved IQ version with IS and USM.