I had never planned on having just one child under my roof... but that's what we have and it's amaaaazing. :) :) :) I'm relatively young, had my son when I was 23, but I'm very much advised not to have anymore (at risk of my own and that child's death. Not just a statistics thing, but a "if you get pregnant again, you'll most likely die" kind of thing. Long story short, it was iffy whether or not I'd survive my son's birth.
Having an only definitely has it practical and cultural challenges (and we're up here in the liberal NW!)... but the reverse is true as well for parents of multiples... they are just different challenges.
People mentioned the "traps" of having an only, and they're real... as real as the "traps" of having multiples. IMHO "only child syndrome" is actually a parenting issue, not a sibling issue. Ditto all the multiple "traps". If you don't want a spoiled child, don't spoil them! If you want a child who shares, that's a LEARNED skill for the vast majority of children. (Our kiddo was a natural share-er until the first time someone took what he was offering to them, knocked him down, hit him with it, and ran away... then it was 2 years of painstaking work. I was ticked -royally- at the time, but it would have happened eventually since we don't keep him in a bubble. My suspicion is that the reason sharing is a learned skill - is just that - we live in a society of people, and interacting with people in a culturally appropriate way is a whole series of learned skills).
The traps people have mentioned have never really been issues in our house. (Of COURSE toddlers are in their own world, where it's all about them them them... that's developmentally standard... it's our job as parents to ease them out of that phase -without crushing them into believing the opposite. Etc.)
The biggest hurdle in our only having one child is actually that most of our friends/ Ds7's friends have several children. It gets very hard to arrange playtime when our friends are still dealing with naps. But I can only imagine that problem is 10x worse when BOTH sets of parents are having to coordinate naps. :)
Loneliness gets brought up very frequently by parents in argument against having only children. Which was a shocker to ME, seeing as how I came from an extended (but close) family that had over 20 children, and I had 3 sibs myself. I was FREQUENTLY, if not mostly, lonely as a child. Yes there were rambunctious super fun times, but that wasn't the norm. The norm was (in talking with my sibs and cousins) that we were most of us lonely most of the time when we were at home (the twins and trips being the sole exceptions). I think many adults forget what it's like being a child... 1st off... once you're in school, unless you're in a one room schoolhouse, multiage classroom, are a twin/trip, or are homeschooled (all minorities) you're in a classroom 6-8 hours a day, and your sibs are nowhere to be seen until you're home. 2nd: Then the vast majority of kids I know/ knew have a couple hours of HW, have activities (away from sibs), and are busy trying to carve out their own identity within their family. 3rd: Just because you're related doesn't mean you'll actually LIKE all of your familiy members. (( As an adult, a person MAY be lucky enough to be close with at least one sib (I know many families who advocate a minimum of 5 kids, just to give their children the CHANCE of developing a life long friendship with at least one sib)... or they may grow up and only see family (maybe) on holidays.))
My own only, is so super social that he complains of being lonely the moment he is alone. He feeds off of the energy of other people, absolutely glows in it, and is one of those naturally charismatic sorts (if in a super goofy way right now, with giant teeth coming in at all angles... oy... it's going to be braces for this one). I am NOT that way, but he is, so I facilitate it to a degree. He has a LOT of time with others (in groups, one on one, in parties) and he's also learning/being taught how to use his time alone.
Parenting is all a balancing act. And no other group (and I'm thinking KKK right now) is as judgmental as other parents. There are about 1001 ways to parent 'correctly'... but the most tolerant of us have our aghast moments, and lets face it, most parents are the opposite of tolerant. (Hmmm... I see I've added myself to the tolerant collumn, not sure how accurate that really is... but I guess I'm writing it, so my ego can put me where it wants.) But seriously, with most parents I've met it's their way or it's "wrong"... ROFL... there's a book called Lipschtick that I read many years ago that says something along the line of "Before kids you had friends because you liked them; After kids you had friends because you liked how they parented." I have found very little to be truer.
So much of life is the luck of the draw, and how you DEAL with what you're dealt with. One child or 10, they're all amazing, and they're all hard. So enjoy. It's your life after all, live it as you see best.