Check out this NY Times article When the Mind Wanders, Happiness Also Strays.
Here are some highlights:

• “I find it kind of weird now to look down a crowded street and realize that half the people aren’t really there,” Dr. Gilbert says.
• Whatever people were doing, whether it was having sex or reading or shopping, they tended to be happier if they focused on the activity instead of thinking about something else.
• In fact, whether and where their minds wandered was a better predictor of happiness than what they were doing.
• “If you ask people to imagine winning the lottery,” Dr. Gilbert says, “they typically talk about the things they would do — ‘I’d go to Italy, I’d buy a boat, I’d lay on the beach’ — and they rarely mention the things they would think. But our data suggest that the location of the body is much less important than the location of the mind, and that the former has surprisingly little influence on the latter. The heart goes where the head takes it, and neither cares much about the whereabouts of the feet.”
• “We see evidence for mind-wandering causing unhappiness, but no evidence for unhappiness causing mind-wandering,”
• There are times, of course, when unpleasant thoughts are the most useful thoughts. “Happiness in the moment is not the only reason to do something,” says Jonathan Schooler, a psychologist at the University of California, Santa Barbara. His research has shown that mind-wandering can lead people to creative solutions of problems, which could make them happier in the long term.
• Over the several months of the study, the more frequent mind-wanderers remained less happy than the rest.