A quick note on my experiences on the first 24 hours with High Sierra. The installation notes provided by Apple seem to indicate that installing High Sierra will consume a bit more space (some 1-odd gigs) compared to that of Sierra. I have also read, however, that some have actually gain some space by converting their drives to APFS. Optimizations (particularly that of Mail storage) seem to be the main reason of this.
In my case, I actually gained some 20 GB of space! I still need to see where this gain come from, though. Is my mailbox such a space hog all this time? In any case, any freed hard disk real estate is always welcome.