My mum says it is like having all the bad bits of siblings and of only children - she had to amuse a small child AND teenagers at once, when the older ones were ready for adventure/beach holidays, she still needed kids stuff for the baby (oh, and she still was called "the baby" by the entire family, until she was 14 and I got pregnant). She says it was a really bad idea.
But then my mum also says that having her children so young was awful, that she threw her life away, she'll never get those years back, etc. She was 22, married, in a secure job and owning a house when she had her eldest- hardly a gymslip mum.
So take what my mum says with a pinch of salt :)
Having been part of a sibling group like that (we were 8 and 6 when my youngest sister was born, all girls) I would probably go for "batches" of children - ie two older, gap, then two younger. Poor baby sister is about to go to uni and has the expectations of parents, older sisters and even a brother in law on her shoulders now- could be better for her if she had someone to share that with.
Did give me an excuse to play with dolls well into my teens though 