OK, this is something I've been meaning to try out for a long time.
With the first patch, results are pretty much the same except for CSS pseudo elements reduced to pseudo classes like ::after -> :after. Browsers shouldn't have any issue with that.
The second patch is using the default preset which does selector merging (should be OK), transforms reducing (translate3d to transform{x,y,z) if shorter; this one we need to verify), completely removes quotes from font names (this should be OK based on the specs, although traditionally we use quotes for spaces in names).
The gain isn't huge, especially if we go with the first patch. Still the fact that the sourcemaps are OS independent is one plus currently compared to clean-css.
/C @ben-eb for any thoughts/suggestions.
PS. The reason I went with the postcss plugin solution is that there is no v4.x of the cssnano available yet.