I know that I'm only a little over a year into my title as "Parent," so perhaps I'm not the best judge of this, but... A recent article in The New Yorker, addressed this very question by looking at numerous studies to determine whether the benefits of having children, outweighs the costs. Most of the studies showed that people with kids are busier and more stressed - leading them to less happy with life than people without kids.
These studies... just didn't resonate with me. Perhaps its my "only YOU can make YOU happy" outlook on life. Perhaps its my "I refuse to be UNhappy attitude"... but parenting... has NOT made me a "less happy" person. Busier, yes. Stressed, yes.