It seems that a major difference between mainstream lenses and 'alternative' lenses like that of the holga, lomo lca, etc is the vignetting. Also pinhole cameras and some super wides have noticeable vignetting.
This certainly creates a look that is dramatic and different than general photography. However I wonder if vignetting is somewhat overrated and perhaps under-criticized.
I'm not sure how yo would get accidental viginetting unless you were stacking filters or using the wrong hood. I would think it would be a straight product of the lens used.
To me it depends on the severity of the vignetting. Mild vignetting like what the Olympus XA & Ricoh GR1 sometimes produce I find quite nice. I also like the effect that Holgas and some other plastic cameras- when shot in B&W.
Severe vignetting to the point where it's like looking through a keyhole however, I'm not that fond of.
I like the vignetting of the XA. When it happens, its not that severe but focuses attention towards the center of the photo, and it looks a lot different than my modern wide angle.
I like controlled vignetting when reproducing some old photos. It makes them look old. More often, I like the opposite effect, but that would be called dodging with a circle cut out (tilted for oval) when printing - the image fades away toward the edges. Also, some pinhole images are very interesting.