We've given them to DS since he was born (and were told to do so). We use the D-Vi-Sol drops by Enfamil. I was told they taste the best.
ETA: He gets 400 iu a day (as recommended).
lucylea:Our pedi recommended it as well, but its a relatively new recommendations as it wasn't recommended for DD three years ago. We use the Carlson drops from Amazon as its only vitamin D, no extra vitamins and no additives.
Our pedi recommended Carlson Drops - we got them at Whole Foods. You can just put one drop on your breast/nipple/pacifier each day - and while they are expensive ($25 at whole foods), they will supposedly last a whole year.