I'm a bit on the fence. I never felt my life was lacking when I was single and childless. However, I also always assumed I would marry and have DC at some stage. So I don't think I would have been content being single and childless indefinitely, although I wasn't waiting for DC to be completely happy.
I love my DC very very much, and cannot even begin to contemplate life without them. As PR says , my love for them is totally unconditional whereas it isn't the same for DH. I would kill or lay down my life for DC, I wouldn't do the same for DH if there was a risk my DC would be left motherless by doing so. My world has shifted on its axis for them and I imagine every decision is tainted by their needs being the ultimate priority. And I want that to be the case - their happiness, well-being and security is genuinely of more importance to me than my own.
If I had never had DC, I know my life would have been fulfilling and joyful nevertheless. Because I am capable of great joy that is not dependent on my DC before they were around and independently of them now. But my greatest joy now does lie within my love for them and relationship with them.
So do they complete my life? No, on balance they don't complete it. Do they enhance it? Yes. But like anything, it isn't one way IMVHO. Having DC has also been extremely negative in some ways - it's been harder work, a bigger adjustment than anticipated, a financial disaster, pushed my relationship with DH to breaking point, and had a v negative impact on my career. But knowing all this, I'd not change having them for anything. And would cope with 10, 100, 1million times the shit aspects to have them. Cos they is awesome, innit?