For me it's been the be all and end all. The big experience of my life. But it's definitely not that way for everyone, and I can think of a million things I would do to fulfil myself if I hadn't had children (heck I can think of a million things I would do THIS EVENING if I hadn't had children!!) :P
But:
It's changed who I am in a way that I like, or possibly revealed aspects of my nature I value.
It's given me something that I think is important to do: I'm a bit of an arts/humanities sort of person, that's where my passions and talents lie, but in all my endeavours in those fields I'd eventually come up a wall of knowing it just didn't MATTER if I write this paper, finish this novel, research this long-dead socialist - I'll enjoy it, it'll be interesting, I might possibly even be able to make a living out of it if I'm lucky, but it won't MATTER - not like space travel and curing cancer matter, both things my brain just isn't cut out to achieve or contribute to. But I know, every morning when I get out of bed to dress her, feed her, entertain and love her - it matters. It is important, essential work that I am the best person to do, and that I'm good at. It never feels pointless, or like I'm faking it, or filling time, even when I'm not enjoying it very much.
I think it depends a lot on where your values lie combined with what gets you going. I've always been about the inner life, feelings, perceptions, the story. These things are inherently useless except to ourselves; so the only energy I can direct outward that both serves my personal passion but also meets my definition of what is of value that I've found so far is parenting. It's like writing the first few chapters of a story that I know will be the most important thing of all to someone - to her. It's creative, and personal, and will have a significant outcome (good or bad!).
I know this is a lot of waffle but I find it hard to justify why i am so into motherhood sometimes as it often seems to get a bit of a sneer from people of my (educated middle class) demographic who don't understand how I can be so committed to and obsessed by something so mundane when I am capable of more 'high-minded' or 'exciting' things.