Such a tough question, because there are so many different factors in family dynamics that will effect whether or not you get along as kids, and in turn as adults. And then spouses and children come into play as adults and that can shift the whole dynamic more than anyone every imagined!
I get along with my siblings. Sure, they still do things that drive me crazy, I'm sure they'd say the same about me. But when we all get together, we enjoy each other's company.
There were times when we were kids when we all loved each other. There were times we all hated each other. There were times when a couple of us stuck together against the others and vice versa. It depending on age, and life and the time.
I was a middle child, and am no worse off for it. In fact, with my own kids I see the benefit to it! All I'd ever thought about what how much you hear about how awful it is, but my middle children/child (depending on head count at the time! lol) have the benefit of getting along with the ones older than them and younger than them, whereas the oldest tend to be more bothered with the youngest and have less in common. Heck, being the middle ground is great! :)
Don't worry about the favortism. Any parent who will say that they've never had a favorite is flat out lying. The catch is that the favorite changes daily. If it's a life long (or even years long) thing-yeah, it's a problem. But geesh, everyone has days where they debate selling one particular child because of their current behavior, and in turn has days when one of their children seems like the perfect child. Some days, my wild and hilarious 3yr old son is my favorite-so full of energy and life and humor. Some days he makes me insane. Some days my 18 month old is my favorite as I watch her learn new things and delight in them! And then there are the days when she won't nap, won't eat and seems stuck in tantrum mode and I wonder if we'll make it to bedtime. She's not my favorite on those days!
The same can be said for any of my other 5 (and I'm sure will be of the one on the way). And my kids all know that when there is a newborn around-it's my favorite. ;)
I think it's great for my kids to see that attention goes through cycles. My 6yr old son had surgery last week, so obviously he is garnering most of the attention as we are working towards his recovery. None of them resent that, because they understand that he needs it right now, and when they need it, it will be theirs-whether that need comes from something serious like his current need, or if that need is in the form of support at an event, help with homework, or just some one on one time to talk and be together. I look at friends who always had all the attention, or whose parents did everything they possibly could to make sure things were perfectly even between them and their siblings, and they struggle when as adults they've had their own children and the attention from friends, family, and even spouse changes focus to the baby instead of them. And in school-you can't always be the #1. Same for work as an adult. Some times you're on top, sometimes someone else is. Life isn't always fair, but you work with what you have, and realize that for the ups there will be downs, and for the downs there will be ups! It all balances out in the end! Better that they understand that as children instead of getting it as a slap in the face as an adolescent or adult.
Frankly, I think we've all been conditioned to worry too much about it! :)