I've only once been terribly disappointed by a mastering job, and afterward, sent the project to our forum's own Brad Blackwood, where it came back sounding great. I've since recommended his services to several people, and they've always been thrilled.
That being said, I've never attended a mastering session and left thinking "Wow, I could never do that!"... It's a pretty straitforward process that requires a different mindset from tracking or mixing. But it's not some black art.