I intentionally put 4.5 years between my two. My oldest is a girl (which I think makes it much easier), and my youngest is a boy. They are now 4.5 and 9.
During pregnancy, it was great! My daughter was old enough to be trusted while I took a short nap. I would sleep on the couch while she watched a show, or she would play on my bedroom floor while I napped.
Once the baby was born, she was old enough to help out some. She could hold a bottle, quiet baby with a pacifier, fetch things for me, etc. She was old enough to be in love with "her" baby and was never jealous of him. Unlike a lot of new moms, with my little helper, I got showers and bathroom time while my daughter babysat for 10 minutes at a time.
They have always played together, and still do. My son loves to join my daughter playing house, Barbies, Doll house, little people, etc. My daughter is always happy to play with my son's toys, and my son will do anything to join his sister. They are not on the same level, but they always make it work. My favorite part of the age spread is that they have NEVER fought physically. My daughter wouldn't dream of hitting her brother because she views him as being a baby/toddler/preschooler, but not a peer. My daughter has been an excellent second set of eyes to watch out for my son, and has made babysitter's lives much easier.
Also, one last note. We realized that with this age spread, you get one-on-one time with the baby while big sis is in Kindergarten, one will finish college before the other starts, and we will never have two teenagers fighting over a car.
To answer your question though, my kids are best friends.