I originally wanted my children close together but we had trouble conceiving. We started when my dd was 9 months old and it took us almost two years to get a bfp. My dd and ds are a little over 3 years apart.
I think there are advantages to both small and larger gaps. In hindsight, I am very happy with my 3 year gap and am planning for another 3 year gap between our #2 and #3. My dd had a very terrible two stage and I don't see how I would have managed with a newborn on top of that--which I could not have foreseen at the time we were TTC. Also, if you work and have to put your children in daycare, it is hell having to pay for two at once, so the longer the gap the less time you have to deal with that. It's also really nice when your older child is old enough to know that a little bro or sis is coming and to share in the excitement. That was the probably the best part of having a 3 year gap for me. Finally, it's also nice to be able to spend more one-on-one time with each child while they are young and to bond with them as individuals.
But really, I don't think you can go wrong whatever gap you end up with. If I had had my way I'm sure I would have made things work and would be very happy with my hoped for 18 month gap too. It seems like it would be really cool for children to have built in playmates right at home. My stepdaughters are close in age and they are so close and I would have loved to have had that for my kids.