the bond between twins (or trips/quads)?

I currently have 21 month old son, with twins on the way (due in September). While my wife and I were at first shocked (and somewhat terrified!) when we found out it was twins, but more recently we are getting excited by the prospect.

I have been wondering about the kind of sibling bond that twins form compared to siblings that are born a few years apart.

I know there are at least a few fathers of twins here. Any stories about the kind of connection/bond that your twins (or triplets, quads) share?
thanks!
-beb