I have a 7 year gap between DD1 and DS1 (who is actually DS2 because we lost a little boy in between there). My youngest 3 are all spaced approx. 20 months apart. I LOVE IT!!!!
We are going through a challenging time with my two boys right now, fighting a lot....BUT, I really think it's an age/stage thing. In almost every way, I am glad to have them close together. I've had two in diapers a couple times, been tandem nursing forever, cosleeping with 1-2 kids, but those have all been beautiful (and yes, challenging) things. These three little ones share so much with each other, love to play and learn and be little children together. DD1 loves them a lot but she's going on 12 years old and really missed out on having a close sibling playmate.
I think when your in the midst of several small children close in age, it can feel like CRAZINESS. But as they get older, you will see the beautiful relationships they have formed and feel that it was worth it. Plus, (and I think this is a lesser point but valid nonetheless), if you're going to be sleep deprived, just commit to being insanely sleep deprived for several years and then be done with it! Same with diapers! I kind of have the attitude that "diapers, many small bodies in bed with me, messes, interruptions, sleep deprivation, etc. are MY LIFE right now! And in a few years, that will all be OVER and we'll be in a new and wonderful world of having the 3 Amigos running around together, best of buds."