That they grow really fast! They are are only cute and fluffy for a few weeks, they grow to full sized in about 10 weeks.
They will desimate your garden if you let them free range, they eat anything and everything and poop everywhere. Ours now have a third of the garden fenced off for themselves (an area of roughly 40 foot by 15 foot) we have three chickens (at the moment! We are looking at getting more) they eat and scratch at the grass, in the winter there is no grass left, just a mud bath, there are no plants left in their part of the garden.
Setting up to have chickens is quite expensive, a new coop can set you back around £150 for 3-4 chikens, then you need feeders, chicken food and bedding, we spent £250 initially, although we have just brought a second hand coop that is virtually brand new for £70. If you don't want them ruining your garden then you need to have a decent sized run for them which should be fox proof (although ours isn't but if we saw any foxes around then we would fox proof it.) They also eat scraps and if you have a compost heap they will flick it everywhere looking for worms and other bugs.
BUT...the eggs are amazing! So tasty. (Although they don't start laying until 17ish weeks which feels like forever when you are waiting for them to start laying) they are such little characters, so funny. Ours come to us and climb on our lap if we sit down in their enclosure, feeding them "treats" is a joy to watch, my dad took out their treat bowl today and they literally danced with him while he was trying to put the bowl down. We love our chooks!