Some time ago, I found this lens at reasonable price and I bought it, just out of curiosity, since it's one of the few EF lenses that I never had the chance to use. Finally, I tested it "on the field" and I'm really impressed. I made a full reportage of a Softair competition using just this lens and it never disappointed me. Unfortunately, since I usually work with video, the jerky focus ring is definitely a "no go" for me, but for pictures, it's definitely a good lens.
At 1.8, the center of the frame is sharp enough (though vignetting is so high that this lens should be considered at least an f/2...). From f/2.8 quality is already good on the whole frame. From f/4 to 8, sharpness is very high from corner to corner, CA is nearly zero and vignetting is negligible. At f/11, even though diffraction comes in, quality is still good on the whole frame. f/22 is still usable.
Distorsion, of course, is well controlled.
Background blur, just like with the 1.4, sometimes can be pretty nervous, but nothing terrible. What's more, I find the pentagonal shape of the f/1.8 iris much more pleasant, smooth and unobtrusive than the octagonal shape of the f/1.4. Foreground blur is always soft and pleasant.
Resistance to flare is average, maybe a little bit below (even considering the simple Gauss...) but nothing to be too worried about.
AF is for sure not at Canon's best, but is accurate, reliable and fast enough even for an event like a softair competition. I shot around 120 pictures and none of them is out of focus. Well, not for an AF fault...
All in all, to me the f/1.8 MKI is still the best alternative to the f/1.2 (which, in my experience, has more CA and less corner sharpness) the true limit of this lens, as far as I'm concerned, is the little, jerky, focus ring. But if you're shooting stills and you use the AF, that's dafinitely a lens to consider.
I hope that Canon will soon come out with a new f/1.8 or f/1.4, with a good focus ring, an IS, a shorter MFD and a circular aperture diaphragm.
Shots here were taken at f/2.8 1250 ISO (even in dim light with a moving subject AF works fine) with a 5D MKII. No sharpening, but images look sharper because of the bicubic downsampling...