It depends who you have and how your parents deal with friction.
Absolutely agree. Have 3 much older siblings - 15-22 years older than me - they were brought up by a mother whose basic parenting theory was 'divide and rule' - they all hate each other. I was brought up pretty much as an only child - indulged, yes, spoilt, no and definitely the most balanced of the four of us (not just my opinion but that of most of my family). I'm the only one who has a relationship with both of my sisters (our brother - another story!!).
I have just one child and whilst I see many of the problems I had repeating themselves - ie, no cousins close in age, an aging family, no grandparents - the advantage he has over so many of his friends with siblings is an 'older' mum who perhaps has a more mature, balanced view on what is important in life and has the time to spend with him, introducing him to a wide variety of interests. I often take my DS to the theatre and concerts and buy 'family tickets' so that we can take some of his friends along - it amazes me how few of them have any cultural experience beyond the cinema and computer games.