Basically what girlsville said- I have a 15 month age gap between DS and DD, they are currently 20 months and 5 months.
DS is very sweet with DD and too young to be jealous as he doesn't know any different tbh. Had same problem when pg that DS didn't start walking until 4 weeks before DD arrived, so was lugging him around still whilst very heavily pg- NOT recommended!
We're lucky that DS sleeps well now and so far DD is a DREAM baby, so easy compared to how DS was and has slept through the night since 9 weeks. Even with her being easy going it's been bloody hard work, and i'll probably look back on this time and think "how the fuck did we manage?!"
Logistically such a small age gap is hard work- I have to go out with them both at least once a day otherwise we all go mad, but this means a ridiculous amount of planning i.e. when will DD need a feed and where can DS be whilst I do this? Also need a crazy amount of stuff when out and about- I doubt i'd need as much for a 6 month tour of Afghanistan
Also, the cost of nappies for two is nuts.
However, I wouldn't change it for the world and I sort of like the fact that we're still in the 'baby zone' when it comes to buggies, nappies, early starts. I think it would've been harder for us to have another one 3/4 years in as i'm not sure I would want to go back to all that.
Hoping that DS and DD will be close growing up, although obviously no guarantees.
Go on, feel the fear and do it 
PS we are not having anymore!