we don't do juice at home, but DD has been getting it (watered down) as a special treat a couple times a month since she was about 18 months old.
she likes it, but i prefer for her to have water.
we do smoothies all the time though! especially if she's not interested in food... she's always up for a smoothie! i put in frozen fruit, wheat germ, some milk and yogurt. i make popsicles by pureeing fresh fruit and water, and sometimes put in yogurt as well.
ETA: DD's pediatrician said that she doesn't recommend juice, but juice that you can't see through is best. orange juice, veg/fruit combos etc.