I have to agree with those who said there is no correct answer. It depends on the family, and the personalities of the children/parents.
For what it's worth, our children are 6.5 years apart, and I love it. I had a very rough first pregnancy/delivery, and was told not to have more children. We thought we were having an only child because of those complications, but we were wrong. We were so pleasantly surprised. :) I was terrified throughout my pregnancy, and had tons of complications, but now we have another healthy son.
I was worried about the big age difference, but we haven't had any problems. There has never been any jealousy, and that's a nice perk having them far apart. I was also worried that they wouldn't be close, but they are extremely close. They are 7 and 14 now, and I figured the 7yo would be irritating the 14yo by now, but so far so good.
They both play well alone, but enjoy playing together, too. I think a lot of it has to do with personality, though. Our oldest is extremely laid back. Nothing ever bothers him; he's happy all the time. If our youngest asks him to play legos, or build a tent, he's game. He never worries about what others might think. Our youngest idolizes his brother, but recognizes his brother's need for space, too.
It's also nice for us because we had so much time with our oldest alone when he was younger. He was an only child for over 6 years. When he goes to college in a few years we will have 6 years of alone time with our youngest. I'm very close to my children, and it will help me adjust to my kids growing up. :)
I've also seen many families with children spaced close together, and it has worked out very well for them, too.