We put our first in at 8 weeks and our second in at 6 weeks (in a preschool-quality daycare). For our first, I think he would've been better off at home - and 2 years later, he's going to get that (my wife is staying at home with them now). He'll go to part-day preschool in a year or two, but mostly will be with mommy. He's very independent, and even in a large daycare didn't really do much with the other kids - he played wherever he wanted, and the other kids either joined or didn't, but he didn't care much.
Our second, about a year old when we took him out of daycare, probably would've been better staying. He is much more social, and even at 8-10 months was playing with other kids more than his older brother was at the same time. He certainly did fine at 6 weeks old and still has a strong bond with his parents.
I suspect in the end it doesn't make a big difference; a good daycare versus a well-organized parent will provide different experiences and different benefits, and one might be better for a particular child, but you can't really go wrong overall as long as you have a good situation with attentive caregivers, and aren't just putting the kids in front of the television.