I have 22mths between the first two and 21mths between the 2nd and 3rd. (so a nearly 4yr old, 2 yr old and 3mth old).

The pro's:
They play so well together.
They get to grow up together.
All of their toys and clothing is interchangable or able to be handed down (although my boys may not appreciate being dressed in their sisters pink clothing as they get older ).
It is fun doing family activities, because they all enjoy the same kind of things.
They help entertain each other, and play games together, which means I don't have to entertain them constantly, freeing up time for other things (like housework, or b/f - so maybe that should be a con ). It makes things alot easier when they are getting along.

Cons:
It is hard work (sometimes REALLY REALLY hard).
it can be stressful running after all of them, or trying to disipline them as they are close in age but still at different stages of development (DD doesn't always understand why she gets in trouble for something DS1 may not).
When they are sick (and you are usually at the same time), it is exhausting and can be hard to cope.
Changing nappies for 2 kids at once is expensive and a pain in the butt.
Trying to TT when you have other kids to look after is laughable.
In my case trying to fit 3 cars seats in a car.

Could go on for ages with the cons, But the truth is that I wouldn't change the age gaps I have and love them being so close together. I love it - but I wouldn't be honest if I didn't tell you that there are alot of drawbacks (I go through periods of really struggling to cope). Generally the pro's (whilst might appear on the list as slightly less, are of more importance then the cons - I have found.

When things are good, they are very, very good. But when they are bad they are horrid.