In our yard they aren't self-watering. They do, however, act as moisture banks. We did our first one in 2010 or so. We hadn't collected nearly enough material (I way underestimated!), but that bed continues to be the go-to bed for plants that need more consistent moisture than my haphazard style generally affords. The soil is ever so slightly warmer (from the slow composting process I understand), and some plants that don't overwinter anywhere else in the yard pop right back up in the spring in that bed (even Datura, which typically [thankfully] doesn't have a prayer of surviving winter in Wyoming).
There are old tree roots under parts of our front lawn. Their locations are revealed as trails of green turf when it's warm, dry and windy here. Expanding on that concept, I burried test plots of the city's single-grind wood mulch under the sod (excavated rather than mounded). Now, in addition to green trails, we have green polka dots. Last September the temperatures stayed warm (something like the second-warmest September on record for us), wind as usual and nary a drop of rain the entire month. The polka dots eventually went dormant, but they were the last places to do so - plus they greened right back up when we got rains in October. Maybe this will be the year I get wood chips burried under all the worst areas of the lawn and lose the polka dots...
