well, there is 4 years between my older sister and myself, then 18 months between me and my younger brother, then 18 months between him and the youngest brother. When we were growing up, my sister really hated me, she was awful when i was born, very jealous, and we never really got on untill she left home and we were both grown up. felt like too big an age gap at the time, whereas, myself and my two younger brothers, were like the best of friends. Of course now, i get on with my sister very well.
I have 17 months between my Son and my daughter...was not how we planned it, but i would not change it now, i dont feel that my son gets less attention since his sister came along, and he accepted her very well, never had any jealousy issued at all, and he is very loving toward her. Im sure it all depends on the child and the parent, have to say, i think for the moment two is my limit, i cant imagine adding a third child into the mix at this moment in time.
Myself and dh have often discussed, how because they are so close in age, holidays, days out etc will be easier as they are of a similar age to enjoy the same things, as oopsed to one being to old for something and being bored, or the other being too young, and being bored.