A.R.
Mine are 18 months apart; they're now 5 (boy) and 3 (girl) and it's wonderful. They get along great, it's so nice to say "go play" and they do - because they have someone to play with! They watch the same shows, play with the same toys, have the same aged friends - it just worked out really well.
Regarding sibling rivalry - we had none. We get some now, but it's more true competition type rivalry and not the mom/dad jealousy type. My son doesn't remember life without his sister, he thinks she's always been here too.
I'm really glad we had them close together, it warms my heart to see how close they are to each other and how much they enjoy each other.