Three years apart has been just lovely for us. DH and his brother were three years apart, and that was great for them, too. They've always gotten along well. It's far enough apart that the "baby" doesn't really expect to do with the big guy does, but it's close enough that he can keep up and they can play together. My sister and I were roughly 1.5y apart, and I think we'd both say that it was too close. We've always had an extreme relationship: either best friends or worst enemies--no in between. It's hard.
However, I will also note that DH and I wanted two kids even before we were married, so for us, the choice was "how close to space them," not so much "should we have another for the sake of the first." On that aspect, I'm with acs: I think the choice to have or not have a second child should be a decision based on many factors, not one made primarily for the sake of the older sibling.
