My DS will be near 5 when this one is born.
We sorta planned it that way. I always knew I did not want a small age gap - it took us a little over a year to finally get a stuck bean but either way, the gap would have been larger than most people we know! lol
This was a very personal decision and what I feel is best for our family. I certainly don't look down on families who plan smaller age gaps - but for us, it felt right because I felt DS needed me before we added to our family. He has had what he needed of me and we are ready to add to our family. I wanted us all to be ready to add to the family - not just want I felt I wanted (cause I was broody for another when he was 3 months old! lol) - I felt I had to consider every member first. Of course he still needs me, but in a very different way than a baby would - and a two year old to me is still very much a baby. I feel really glad that he has had that time needed of me and he won't be pushed aside. Hes a little over four now and can be patient, wait happily, doesn't have any immediate needs that can't be waited for, pretty self sufficient in ways that will make a difference when I have a small life to look after that does have real immediate needs, etc. All of these things will make it easier on all of us when it comes to the practical stuff - and the not so practical everyday applications, well - another bonus is that I can actually have a conversation with this kid now and he has understanding and is putting together logic skills as well - so much different from when he was 3 or less! hehe.
At the end of the day, I did not choose to have another child to be my sons best friend. He has his own friends. If they get on, thats great! And if they don't, its not the end of the world - he does have his own world and that includes friends, so there won't be anything lost if things don't work out perfect family and only things gained if it works out wonderfully! How your childred do or don't get on is never anything you can predict in all honesty - I personally feel it would be naive to assume that. However, saying that, I have a handful of friends who do have a 5+ year age gap and all of their children get on wonderfully! From my own observations, more so than those with much smaller age gaps - and I think this has a lot to do with both not having to share the one thing they need the most because a five year old no longer needs mummy the most and has the understanding to know what a baby needs and can talk about their feelings/emotions through any bumps during that time.
I personally have never seen anything negative about a big age gap. I think its all about expectation. Have some common sense about the everyday stuff - and the rest can only be a nice surprise! lol