My kids, when it is/was age appropriate of course, know most of what there is to know about what goes on, what went on, what things were like when we were younger (including old bad habits, yet).
I don't share things that I feel would burden them, stress them out, cause them worry. I think there is a balance. So while I would say "We have to cut back on extra spending for now" it's different from "we are in big financial trouble and I don't know what we're gonna do!", to give a bad example.
Marital issues? We're not going to pretend to be always happy and act like there are never problems, but we don't want to worry or stress them with fighting either. If we're having a disagreement and one of them notices that there is tension then yeah, either of us might acknowledge that yes, we're having a spat, but that it will be okay because we're going to work through it. And with consideration again for age-appropriateness we might say "Now I was being a jerk, that's an example of how not to behave..". I think it's important to let them know that married people can get mad and really NOT like each other at the time but still work on their problems and come to a peaceful resolution, vs. pretending everything is hunky dory.
I was full of "old bad habits" before having kids (drugs, alcohol, promiscuity) and I don't try and hide this or minimize it. I don't talk about it all the time but the older 2 know that I was a very different person before having kids and I try and stress the fact that I really did narrowly escape a very different fate. I want them to be aware of how easily I could have ended up in jail, dead, or worse. I know they are going to make their own choices and not necessarily base their decisions on my own history but I don't want to sugar coat things and I don't want them to minimize the dangers when they are making their decisions.
So far so good, they are 20 and 15 and none have been anywhere near the "problem" I was by age 13, 14, 15! Oldest has only had one or two "drinking incidents" and won't touch anything else, and seems to have learned from those two drinking binges that it's not something he wants to do. Younger one seems to still be on a smart, sober path and doesn't hang with anyone with bad habits either.
I had a friend who went out of her way to paint herself as some sort of June Cleaver perfect mom to her kids (she and I were friends since Junior High so there was NO truth to that act, trust me!
and she would even go so far as to get ready for a "night out" away from her house by getting ready somewhere else. She and her husband would sometimes go out clubbing where she'd dress sexily/provocatively but she was so hung up on her kids not seeing her in 'that way' that she would take clothes with her and get dressed elsewhere before leaving the kids with a sitter. She thought it was wrong for the kids to see their mom looking "sexy" in any way.
She had not changed in many ways, she just kind of suppressed her more "wild side" and as a result the kids had no idea who she really was as a person... when she had something of a mid-life crisis years later that involved her moving across the country for a much younger guy she'd met online, leaving the kids with her ex-husband, they were absolutely shocked. She'd built up such a false image as the ideal soccer mom/cookie baking Pollyanna, it was as shocking to them as when some TV preacher gets caught with a male escort or whatever.
Hope I am not rambling too much with that story to illustrate my point. In later talks with her oldest daughter (who is one year older than my oldest son) she had no idea her mom had ever done anything "bad" as a teenager, her mom had always pretended she'd been perfect and pure. I think that's kind of messed up. Like I said, balance is needed.
My kids know that I recognize some of the bad choices I've made and I hope they won't make the same bad choices because they have had honest discussions with me about that sort of thing.