Personally, I hate them.
The Private Catholic School I went to from 4th til 7th grade required uniforms. It doesn't matter if you're all dressed the same. There's still the "who looks better" stuff, the making fun because someone's uniform dress skirt/jumper is longer and to their knees or more than the other girls. Kids, boy or girl, will be mean, regardless of whether or not you're all dressed the same. We all resented having to wear the uniforms. The few Free Dress Days we had were awesome. Not to mention, the uniforms were expensive. Only certain shops had them, and so it was a rush to get the right sizes in time while EVERYONE ELSE was at that exact same set of stores doing the exact same thing as you. I don't know how all uniform stores are, but I know it was 20-30 dollars per skirt, more for slacks (which the girls were definitely not allowed to wear, so if you were uncomfortable walking around in a skirt, too damn bad), and polos were 20-40 depending on your grade's color. I would not choose a school that required a uniform.