I have two teenagers, 17 and 18 years old. For us, the stress on the marriage directly correlates to the amount of care needed by the kids. It was bad at the beginning, when there were two in diapers, then got a little better as the kids got a little older. When we started having trouble with our son in school (the highlight being the police bringing him home when he ran away from home - when he was about 8 or 9), it went downhill again, but now that the kids are pretty self-sufficient it’s not too bad. We’ll see what happens when they move out and leave us in peace - maybe it will be like a second honeymoon!
I agree with the people that have said it depends on the strength of the marriage. Our marriage is not the best, and we disagree a lot on child rearing issues. If we had a united front, I think it would have been easier, both for us and the kids.