I've really enjoyed reading this thread. We have an only daughter, age 6. For us, it is has been great. DW is also an only child, so it has always felt normal to her. I have two siblings, who I'm close to in the sense that we have a good relationship, but I left my hometown (and country) 20+ years ago so my actual interactions with them have been very limited for a long time. And, we're all pretty terrible about picking up the phone.
There are some things, though, that we've been intentional about with DD being an only. For one thing, when we decided to move to Canada, that decision was partly informed by DW wanting to be somewhere that DD could truly feel like the belonged. We're originally American, and I lived for most of my 20s in Taiwan and China. While I enjoyed those places, expat kids who were born there (in my experience) were never truly considered to be of that place. Similarly, even though we are both trained as ESL teachers, we didn't consider bouncing around from country to country every couple of years (even though we'd enjoy that) because we wanted our daughter to be able to develop long term peer relationships. If she had a bunch of siblings, we might have felt differently.
And, this summer, even though DW was mostly not working, we put DD in day camp most weeks, just for the peer interaction. She absolutely loves it. We moved to this town in Canada last summer, and I joke with her that she's the mayor because she knows everyone -- she has friends from school, from camp, from soccer. Whereas me, as a work from home (for a US company) introvert, I know virtually no one. We live in a small town in Nova Scotia, and I love the fact that she's going to be seeing a lot of these same kids in various contexts for the next 12 (or more) years.
I also feel like we have a great family dynamic with just the three of us -- she sometimes requires more attention (both conversationally and otherwise) than an adult would, and we're able to give it to her consistently because she's the only kid. I also feel like her verbal skills (both speaking and reading) are crazy high because she has so much experience interacting with adults (although some of that could just be her). I also appreciate the convenience / cost benefits -- in terms of flights, hotel rooms, etc. That's pretty small potatoes, though.
Our daughter has never (literally, I think) asked for a sibling, which is kind of interesting. She definitely went through phases where she loved playing with baby dolls (and actual neighborhood babies) but I don't remember her ever asking for one.
And as other have shared -- I do sometimes feel a bit bad about her not having the built in long term relationship of siblings. At the same time, those can really go sideways. And, even when they're good, they can sometimes (like mine) not ultimately be all that important. Bigger picture, I think I sometimes feel guilty about not giving her more time with family in general. At the same time, I love where we live, and it isn't near family. Plus, some of our extended family dynamics are a bit complicated.
Ultimately, though, this wasn't like a master plan on our part. We were old -- I was 39, DW was 35 -- when we had our daughter. If we would have been younger, I suspect we would have had another. At the same time, I don't think any one of us feels deprived or that we're missing out. Being a parent is awesome and I love our family just as it is.