I met my DP three years ago. He already had a DS, who was 4 at the time. DS and his ex split up when DS was 1, so he doesn't really remember them being together.
Although DP and I are not married, I am in effect a stepmother to my DSS, who is now 7. Sure, it isn't easy all the time - it's hard sometimes dealing with the negative points of having a child around without getting the good stuff of unconditional love and affection.
But.
DP and his ex worked hard to put their problems as a couple behind them and remain on good terms with each other. It has really worked.
His ex met a new guy quite quickly, and has since had another child with him. He already had a daughter from his previous relationship. So they have three children at their place, albeit on a sort of rotating shift system!
DP and I, and his ex and her DP are all on excellent terms. We do stuff together like dinners, DSS's birthday, summer BBQs - about once every couple of months. All the various incarnations of kids/step-kids/half-sisters/half-cousins are running around, and I love it. I particularly love DP's ex+new partner's little girl, who is just adorable and we have a nice bond between us.
I'd say that while I do still struggle sometimes with specifically being a stepmother, and of course DSS sometimes wishes his parents were still together, overall I'd say we're all doing pretty well. I actually really like being part of a wider blended family and it CAN work and be a really rewarding and special experience. I feel quite proud of what we've all managed to achieve together.
Of course, the story of our blended family isn't over yet - DP and I haven't had kids of our own yet, so that will probably throw up a whole load of new issues if/when it happens. But only time will tell...