My older SD is now almost 9 (in just three weeks!) and younger SD is 5, I moved in with their father when they where 6 and 2, and was worried about how they'd feel from the very start. Well, fast forward, I was lucky, it was love at first sight with them. Now, younger SD isn't hubby's bio-daughter, but he's never rejected her b/c of that and has obviously bee around since before her birth.
But hubby's made the HUGE mistake in my opinion, he's followed the kids around since the marriage broke up. She moves once a year at least, but sometimes (like this school year) 3x or more times, usually out of state moves each time. We even did this once after we became serious, but NOT since because it was only three months after begging for us to move down there and help out with the kids (she was having some problems, they stayed with us) that she got a new bf and moved BACK to where we'd moved from.
I love them JUST as much as my own, they are mine too, even if its "just" as SDs, and miss them every day but changing jobs that much, spending the money on moving etc it means STAYING always broke, always paycheck to paycheck barely getting by. I beg all dads and stepmoms to really really think before they do this.