I have a 4yo with my husband who has 2 children from a previous marriage. At the time of birth his older kids were roughly 10 and 8. Please keep in mind that my situation isn't typical. My husband and his ex can barely be in the same room together, she's violent and, well- crazy. She continually keeps the children from him so we only see them a few times a year for a weekend. My husband also works an extreme amount so he is stressed and tired a lot. Anyway.
Do I regret my son, never, he's one of the best things to happen to me. Do I regret having my first child with someone who has children in his life already, yes. Everything has been experienced on his part already. Nothing is new or exciting because it's been done/had/heard.
I was overjoyed with pretty much everything my son did, learned, said, etc. "Look, he's smiling... look, he took a step... look at that face he's making... look, he made a mess!" The first holiday's and first birthdays... I wanted to document and save EVERYTHING.
But... more often then not everything was "First child did this... said that... Oh, second child did this..." I didn't feel my husband was "into" everything because he had done it all before.
Because of the situation between my husband and his ex and the rarity of seeing his kids, I felt any visit was focused too much on them. For example, biomom "allowed" us to have the kids for what turned out to be my son's first Christmas. In hindsight, I think she did this simply because she knew it would ruin Christmas. My husband broke himself to afford gifts for them, we stressed ourselves saying "is this enough for them, will they like it" etc. Instead of focusing on MY son, buying presents on him, I was too worried about how his children would handle things. And it was horrid. Both actually said the phrase "Is this it? I wanted more. I didn't want any of this." And then, of course, a 2 hour drive to return them. On Christmas.
And my son's 1st birthday was planned around when biomom would agree to allow the kids to come up.
Now that my son is 4 and talking he is realizing he has brothers and is constantly asking for them to come over. Why do they have to leave, can they come tomorrow? Tonight he started cleaning his toys up and when I asked him why he said, "So D and L can come tomorrow!" It broke my heart. He will never have a true relationship with his brothers because biomom won't even allow a relationship between them and dad. And that effects MY son.
I feel that no matter what my son does/says it will be compared to his older kids in some way by him. Which happens in families, anyways, I feel, but it's different because he's my first (and only) and I want things to be special and I just feel they only are to me.
As I said, it won't be this way for everyone. If you are with a man who has children and he coparents great with the biomom it can be wonderful. Your child gets brothers/sisters but also gets to be an only child.
If I had it to do over again- I would not choose to even BE in a relationship with a man who has children and wasn't in a good parenting relationship with the mother. I certainly would not have children with him.