To expand a little on what BB said, raisins give the mead a little broader mouthfeel, but add a somewhat vinous flavor, almost a slightly spice slighly acidic character. I like raisins and have used them on and off in my meads, but it seems in the past couple of years that my tastes have changed somewhat. Also the regular raisins color your mead a bit darker, and there are some folks on the forums here who use the white or red raisins.
At this point I prefer dried cherries, currants, blueberries and cranberries to raisins. If' I'm going to make a "natural" mead using other fruit in the primary as nutrient, I prefer to use one that will positively influence the flavor of my end product to something that I prefer over the character imparted by raisins.
Hope that helps,
Oskaar