Having been a stepkid myself, I'd like to remind everyone that it's no picnic from the kids' point of view either. It's no fun to have your parents split up (even if you're old enough to realize it's for the best, and even if you don't harbor a secret hope that your parents'll get back together). It's no fun to have them fuss and fight over money, time with you, holidays, and more. It's no fun to listen to your custodial parent or grandparents discuss how badly one of your parents is treating you -- after all, you still love that parent.
It's no fun to be dragged away for a visitation weekend when there's something really great going on at your school or in Girl Scouts. It's no fun to have to stay in a room that isn't "yours" -- even if everyone is really nice, and even if you get to go out and do all sorts of fun stuff together, you still feel like a guest during visitations. It's hard to have two sets of rules, even for little things like way food is prepared or helping clean up. There's a transition time every time you go from one house to another, and during that time you don't feel like you belong at either place.
If you're one of those kids who's learned to "play one parent against the other" and get everything you want -- whether it's permission to go here and there, or whether it's material goods -- well, then, you have a whole other set of issues. It's nice to think that you can get what you want, but it's scarey to think that you have power over your parents. You want your parents to be strong and in control, but if you can whine and manipulate them, it's scarey to you.
And you -- the child -- have no say-so whatsoever in these things! You probably didn't want your parents to divorce. You probably didn't want them to remarry. You cannot control much of anything in your own life.
What's the point? While I'm sure being a step-parent is really, really difficult, you have the benefit of being an adult. The kids have it just as hard, but they're coming at it from an immature perspective. This doesn't make the step-parent's job any easier, but it is important to remember.