I find most stories about human-like robots (e.g. Pluto) dreadful, and the whole notion of Slice of Life as a genre bothers me, since it seems to presuppose that Random Stuff needs no skillful handling in order to become art. These two concerns about YKK (it was originally billed above, for example, as a "slice of life" manga about a "robot ... that appears and acts fully human") kept me away from YKK for quite a while.
I finally gave it a shot because of barbapapa's list in the Top 5 Favorite Manga forum thread, where it's counted along with some other titles I like; and I'm very glad I did. In that posting he parenthetically notes that YKK is "better than life," and that is exactly it. This manga is better than real life.
As far as my fears were concerned ... One, the story never goes in for the whole manufactured robot?/human?/what-is-life? drama in the first place (it simply doesn't go there). Two, it emphatically doesn't belong in the Random Stuff bucket. Long episodes of house repairs and brewing coffee substitutes might seem, at first blush, to be the essence of Slice of Life in that sense. But as you read on you begin to see how these mark the deep beat of an epic poem on growth and decay, built from a handful of delicate but expertly handled twists on convention (it's the story of the growth and decay of species, rather than of individuals; decay is the primary theme, and growth the secondary one, rather than the other way around; the immortal god-like narrator figure has been flipped from its usual post outside the story to its center, in Alpha; etc.). So far from being random samplings, they're carefully crafted motifs in a much bigger picture. The result is a masterpiece.
One more thing. I don't know anything about the publication history of YKK, but it has the feel of a story that was cut off before the author was really done with it. It seemed to me that volume 14 ends abruptly, and then he gets the space of one very short Epilogue chapter to wrap things up. But the Epilogue isn't one of these inadequate patchwork wrapups at all --- on the contrary, it's an amazing ending. One of my favorite manga endings ever. Just one more thing to love about it.