We started TTCing DD2 when DD1 was 15months, fell 2nd cycle and she was born 2 weeks before DD1 turned 2. Absolute nightmare age gap for me since DD1 didn't toilet train (or wish to) until after 3, was not ready for a bed when I needed her to go in one which involved many hours of patting and fighting sleep, chalk and cheese personalities, never would nap at the same time, they really aren't friends even now and fight more than they get along... I wish I had waited at least an extra year, but it seemed like a good idea at the time - everyone said they would be the best of friends when I found out I was having another girl and that the age gap was "perfect" lol
Girls were 5.5 & 3.5 when we started TTC DS1 (took 5 months this time) and they've just turned 6 & 4 a few weeks ago and it will be a much better age gap come October as DD1 is now at school and DD2 can pretty much look after herself, get herself food, take herself to the toilet, change her own dvds, play by herself if need be which has been great during bouts of morning sickness and full on fatigue and she'll be starting school next year.
There is 4.5 years between my brother and I and I remember my dad saying it was great because of all the reasons I'm finding with DD2 now.