My family and I have taken two one-month vacations and numerous shorter ones with one carry-on each. I traveled a week in Germany with 1 1/2 changes of clothes in a daypack (and I'm working my way back to that).
After each trip, I ask myself, "How could I have taken less?" It is so much nicer not lugging around extra weight, especially when you are using public transportation. Checking luggage is a hassle. I like to hit the ground running (so to speak) when I land and it's nice not to worry about lost items. If something was lost, I wouldn't know where to forward it to because I don't travel with a set in stone itenerary.
If your clothes are versatile, layerable, easily laundered, relatively wrinkle free, and everything mixes and matches, it's amazing how many looks you can get out of a small bag.