Mine are 21 months apart. It's a little closer together than we would have liked, but not by much. (Conceiving the second was a lot easier than conceiving the first, and I'm told that is common. In fact, it couldn't have been any easier.)
The down side is that a 21 month old is still pretty needy. Trying to be a good parent to a young toddler and an infant is extremely demanding. When people ask me about the demands of having a second child, I usually respond like this:
How much time do you spend taking care of your first child?
How much spare time do you have?
Everyone seems to realize that taking care of a child requires more time than they have to spare. That means that things will get cut out of your schedule. For instance, I haven't shaved in a week. I have a bunch of laundry piled up waiting to be put away, etc.
Luckily, my daughter (the introverted elder child) has always been good at playing by herself, and likes to help care for her little brother. She "helps" give him baths for instance. I still try to make as much Daddy-Daughter time as I can, but it's not as much as it used to be.
I don't have research on this topic, but it seems to me that children closer in age are more likely to form closer bonds. They can play together because they actually like the same things. When one child is too much older than the other, then they play with their younger sibling the way we play with our kids. At that point it's more about spending time with your loved one than participating in an activity that's actually fun for you. Thus, there's a level of excitement that's missing because there are few things that both children will agree are totally awesome.
It seems to me that once the mother and her womb have recovered, if you're willing to put yourself through the difficulty, then go for it. If you want it to be a bit easier, then time it so that the first one is out of diapers before the second one arrives.
