Two years ago I married a woman with a two-year old daughter. We hit it off pretty well, so the fourth date was at a folk music festival and I met Cori for the first time. I was introduced pretty much as a friend and kept any affection to a minimum. It was a test both of what Cori thought of me and how I got along w/ Cori. (I have a sister who's ten years younger than I am so being around little girls wasn't something new). Jen and I took it slow at the beginning, but as time went by, I was around more. It was made clear that her dad (who is very involved in her life) was always going to be her dad, but her mom and I were going to live together and that she could trust me.
All in all it's worked very well. Sometimes it's frustrating because while I'm involved in the family decisions from her mom's end, not all the decisions they make are the ones I would. (since her dad has input on things). But I surely wouldn't change things.