I had 3 under 3 - there's 15 months between the first two and 16 months between the second.
It's possibly the most mind-blowingly difficult and yet completely fantastic thing I've ever done in my life, and if I had my time over again, I think I'd probably go for the same gap.
The low points:
I did feel a bit as though I was missing out on dd's babyhood as I was already expecting ds1 by the time she was 7 months old. I was also still bf dd so that was quite hard to combine with the early stages of being pg, although there's a lot of advice out there if that applies to you. In the late stages, it was hard work running round after a toddler while being the size of a whale myself, but it doesn't last forever.
None of my children have been great sleepers so it was absolutely knackering - I know some people manage to synchronise their daytime sleeps but I never achieved that with any of them! Oh, and obviously you're in double buggy territory, although we managed two overseas holidays with a 1 year old and a baby, so it can't have been that bad! The other thing I remember is being asked constantly whether it was planned, which I thought was incredibly rude.
The high points:
All of mine, but especially the older two, are really really close. None of them have any memory of a life without each other, and it's lovely watching how closely bonded they are. It's also pretty easy to entertain them now they're older as they're all at the same stage at the same time so they enjoy the same books / films / days out.
It also means you get the baby bit out of the way in one fell swoop. Although it is, as my DH calls it, 'extreme parenting', it does mean that you're not trying to manage the school run with a new baby, for example.
I wrote an item for our local NCT magazine about this very subject, so if you're in need of more convincing / dissuading, then CAT me and I'll willingly send it to you!