We have a 21 month gap here and it is FANTASTIC!!! I don't think I could have timed it better!!
DS was old enough to help me out with things (like getting mummy nappies), but young enough not to get jealous at all. No outbursts from him ever. No clinginess towards me because there was a new baby. My daughter just fit right into the family like she had always been here. Was VERY easy from day 1! It's a little harder now that I have a boy who's just turned 3 and a 15&1/2 month old who is the biggest risk taker ever, but they are such great friends and have been from day dot. We are so glad we had our kids close together.
I totally recommend a small age gap. Love it!