We have friends here who turn up unannounced and I both love and hate it!
We open the door with a smile on our faces and a groan in our hearts lol whilst trying to mentally rearrange what we were intending to do (even if it's just chill with our often-invisible kids!).
There are so many demands on our time and being time-poor means we usually have something planned at any given time so although we always enjoy the company of our unexpected visitors, sometimes it's nicer to have a 'proper' visit arranged.
That said, I do like my just-popped-in-for-a-quick-coffee visitors and even though they will never read this, I miss them when a weekend goes by without seeing them!