I think it REALLY changes your life. For me, it was about just not having the same sort of freedom as before. Like you can't just do what you want. You can't sleep when you want. There's little staying up late at night anymore (or at least very rarely) because I have to be up at 6am pretty much every day, 365 days a year. If we are sick and feverish and vomiting, we still have to get up at 6am. If I had a big project to finish at work and stayed up til 2am, still up at 6am.
You can't go and do things whenever you want to. Like leaving the house in the evenings. It's so rare that either of us is not home by 6pm. On rare occasions when we, say, get a take away, I usually drive to go get it (we live too far for delivery) and it's now like the most exciting thing to actually go out after dark! We're both almost always home at that time as we need to get our dd dinner and a bath and to bed (because we do relish the 2 hours of quiet, adult time we get in the evenings before we go to bed). So like, if I'm in the car and driving somewhere at 7 in the evening, it honestly feels pretty luxurious now! That sounds really silly and I couldn't have imagined feeling that way before kids, but it's so rare to leave the house at times like that.
And your time just isn't your own anymore. Before we used to spend our weekends taking walks or going to the pub or I'd go for a long run most of Saturday morning. But it's hard to do those things now. Weekends are mostly cleaning and running around and doing the food shopping and family activities. It's rare we'd go out for dinner or meet friends for drinks as one of us has to be at home. We do on occasion meet friends for drinks still, but it's hard to go out and do things as a couple (so like individually we make plans to see a friend, but it's really only a few times a year now as our friends equally are busy and home with their kids and have no childcare). Just the logistics of getting out and doing things are difficult. I imagine maybe it's easier if you have family around to help, but we don't. It's just us, unless we pay a babysitter and that's more a treat than a regular occurrence.
So that makes it sound awful, doesn't it?! I absolutely love my kids and I couldn't imagine life without them, but it does change things and for me it was about not having the freedom to do what I wanted when I wanted. I think that probably does get easier though when kids get older. It's really only when they are little that it's hard in that way (my eldest is 5 now and I can see how in another few years things with her would be easier). Certainly the parents I know with teenagers have lots of 'freedom' again, but probably other stresses. For me though, the key was living life before I had kids and being ready to make a huge life change. Waiting until I was older really helped I think. I had my first at 32 and 2nd at 37. I truly lived life up until then. I partied, I worked hard, I traveled, I lived abroad, etc. I did everything I wanted to do to be really selfish, so it wasn't so hard to shift life to being a bit quieter because I don't feel like I missed out on anything when I was young. I definitely wouldn't have wanted to have kids sooner than I did, but that's just me.