I was on the fence about kids, but my DH wanted them, so we did. Now they are 12 and 9. There were many ups and downs, but at this particular moment in time, I'm really glad I did have them. On the other hand, I probably wouldn't have really understood what I was missing if I hadn't.
I guess if I didn't have kids, I would want to have something else "worthy" in my life, that was pushing me out of my comfort zone. It's not just about being comfortable and doing things you like. People need to be challenged to be happy, and having kids is that built-in source of challenge that brings amazing rewards.
But, again, I don't know how things will change again as they grow and leave home. Maybe when they go off to college I'll be so miserable, I wish I never had them.

Or maybe it'll feel wonderful.
In general, I would say that you may be happy or unhappy regardless of your choice. Kids bring so much work/worry and change, but also joy you didn't know you could experience. But either way, your life will probably include joy/challenges, in other areas. I guess I would just say that if you're overextended now, it'll be more difficult to add kids to the mix.
Oh... And there is also the biological clock thing. Some women do wake up one day in mid-40s and experience a physical need for children. Some men too. I knew a couple like that. They ended up experiencing infertility and then an amicable divorce. I'm not mentioning this to scare you into having kids, I personally don't care either way... But it does happen.