I have a 16mth gap between my two, so I was already pregnant when ds was 8mths! It was an accident . It was very hard for the first two years, I'd say, but that's really because I'm not great with babies, and dd was quite demanding (ds was a pretty easy baby - hence time for an accident!).
On the plus side, we had no sibling rivalry, as ds didn't really notice dd ariving, and has now forgotten what it was like without her. It was also quite a cheap option, as we already had a nanny, so no extra costs for the first few years. Much more complicated when ds started school, but dd goes there in Sept so that will make life easier again.
The best thing is that they are very close and spend most of the time playing together, so parenting is much easier now. We never have to worry about ds hurting dd as they are pretty much the same size. They also like to do the same things, and have pretty much the same access to things now (holiday clubs etc). We did have a double buggy for a while, followed by buggy board, and two lots of nappies etc. But then we also got rid of all of the baby stuff at once.
I have a four year gap between me and my nearest sister, and didn't have anything in common from when she went to big school until we both had kids at the same point, so I really didn't want a big gap. On the other hand my dh has a five year gap to his nearest sister and they were close as kids.
I have heard that a two - three year gap is the hardest (hitting terrible two's I guess), and I think that it also really depends if you are just going to have the two, or if you are going to have a bigger family.
It's horses for courses really. I'd recommend just seeing what happens, then you won't feel that if it doesn't work out perfectly you are to blame!