I'm a lvl 40 Elemental specced Shaman, but I've successfully healed a couple of raids to mara with good tanks. However, the other raid, I had to leave for quite a while (in game) due to a silly mistake, and while watching the health bars, the other DPS who was substitute healer was doing a much better job, and the mediocre tank didn't have as much health, and after that I lost confidence and stopped signing up as heals on Dungeon Finder.