This answer about a lens adapter with a mechanical aperture control (like these 1,2) got me thinking — Is it possible to control the aperture of a lens in this way? To me the answers to this question suggest it won't work very well at all since the diaphragm is sitting way off where it should be. Also will all lenses work equally well/bad? What determines if an adapter like this is possible to use?
Adapters with a built in iris will work reasonably well but with more vignetting and potentially a slight variation in aperture across the frame. How much will depend on where the exit pupil is, longer exit pupils (typically found in telephotos but not always) will fare better.
Since these adapters are considerably cheaper than full electronic adapters it's probably a reasonable price/performance trade of in most cases, and sometimes it's the only option.
Lens design texts often suggest that for some designs, the position of the aperture is relatively critical with regards to vignetting and distortion. However, an MFT camera with a full frame lens only uses the center bit of the imaging circle, so if the adapter causes grave problems in the corners and sides, these problems might be mitigated/attenuated enough in the center part to be acceptable.