We've been really lucky in this regard - both DP and I agree completely on our style of parenting - he obviously adores his son, but he is by no stretch the be all and end all of the household - he has that at his mothers and DP has seen just how destructive it can be. DP and I treat DSS very much like an equal, and expect the same back - we try to run a very cooperative household whereby nobody is more important than the other and everybody deserves attention and love in equal measures.
I was very lucky that my DP was not a DisneyDad - in his previous relationship he was very undermined in his role and told repeatedly that what he was doing (or not doing) wasn't good enough. When we got together, he had an enormous amount of guilt believing he had been a poor father. However, through lots of talking and some very rough patches, he realised he was never a bad Dad, just him and his ex were VERY different when it came to parenting his son. Initially DP was very incline to over compensate for the break up - toys, bending over backwards for ex (well, for son, but ex if you see what I mean) and generally putting all other needs on the backburner - all that caused was immense amount of stress and unhappiness for him. It was at a tipping point when he was travelling constantly at the weekend to get a few hours with his son just because ex was saying "You left, you come to me if you want to see him" (it was a mutual split, but he was the one who had to leave the family home).
Once left the relationship, he could see just how important his parenting style was and just how much justified he was in causing conflict within the household - DSS can be, sadly, incredibly spoilt, rude and entitled with very little gratitude. However, he is a wonderfully smart, intuitive little boy who despite his attitude sometimes, DOES realise that what DP is fair, even if it takes us a while to get there. Thankfully, DP's ex now is far more accommodating to us and after a lot of hard work, she rarely pulls the victim card out on DP - before, it was constant, and the boundaries were so fucking messed up it ended up with me being dragged into it and spat out. It's taken a good 18 months, and we know there are hard roads ahead (I'm 9 weeks pregnant, plus DP and ex still have a mortgage on the house she's living in and kind of refuses to sell) but I think we're better prepared. His ex has dropped the passive aggressive bullshit to an extent, and has grown up a bit rather than using access and DP's parenting of his son as an excuse for a stab in the back. DP struggles sometimes, mainly as he wishes DSS could be here all the time, and also he struggled with the disappointment he felt towards his ex, but all of these incredibly difficult situations were made so much easier by the fact DP and I parent so similarly, therefore DP was able to approach being a Dad to his son in a supportive, cooperative environment. We've rarely come to blows about whether or not we're putting DSS first, because we try to put all of us first - and DP is definitely not one of those "oooh how could you" type people - he is now very relaxed about parenthood despite the immense amount of challenges he faced early on.
Have I ever felt in competition with my DSS? Nope - again, very lucky so far, me and DSS have a very good relationship - if this was different I could see why people feel an element of competition for attention, but I've always treated my DSS like a friend. I don't coo over him or try to mother him and I step back - I give him medicine and soothe him when he's poorly, and he knows he can always come to me, but I have never insisted he cuddles me, kisses me or tells me he loves me despite being in his life for a long time. Any occasion we've been out and somebody has referred to me as his Mum, I'll always say "Stepmum" (but only since he's started referring to me as this) or "his Daddy's girlfriend" or sometimes "friend" - I've always been very keen to ensure he knows I'm not trying to take anything from his Mum, even if she and DP (and I) disagree. I may disagree with the way she's treated DP and I (that's another long story) but she is my DSS Mother and I respect her for that.
Believe me though, it's taken SUCH a long time to feel this sense of ease and it's definitely not over yet - I'm thinking the calm before the storm right now, but least I know DP and I are better prepared to deal with the shit storm 