I'm an only child. I was also the only girl of my age in my neighborhood. (Lots of boys though.)
Didn't really like it - it would have been nice to have a little sister, or an older brother to beat up anyone at school who bothered me
But it did mean I got my own room, no questions asked.
Was I lonely? Kind of. I had my friends, but none of them lived within walking distance, so planning play dates took some work.
Was I spoiled? In some ways. My parents did spend quite a bit of money on me, but also disciplined me severely - sometimes for stupid stuff - and were very reluctant to grant me permission to do things like wear makeup, carry a house key, or drive the car (I wasn't allowed near the driver's seat until I'd had my license (not my permit) for a couple of months!)
No adopted or step sibs.
In the real world - it was a little strange suddenly living with so many strangers my age, and being away from my parents (I'd never been away from them for longer than a week and a half at a time) but I got used to it pretty quickly (all except my roommates from hell - but that got handled when I lucked into a single, hehe)