My keezer is set to 39°. It seems to be a good compromise of temperature for the variety of styles I serve. Sometime I should measure the temperature at which the beer actually comes out of the tap--I suspect there is a bit of variation depending on where the keg is in the keezer, and how full the keg is (I use floating dip tubes). I have a small fan in the keezer to circulate air and improve temperature consistency, too.
In the end, I don't worry about precise serving temperature too awful much, as long as it's not ice cold for most of the beers I serve. As others have related in this thread, I personally find it really enjoyable to savor how a beer changes as its temperature changes. At risk of being incredibly insufferable, I find that glassware shape is (for me) a big part of enhancing that enjoyment along the temperature gradient...but that's an oft-discussed topic for a different thread!