I've been researching the effects of non-maternal child care on children, to find out that for the AVERAGE child, the type of child care does not matter (so long as it's high quality: obviously, warehousing children in front of Jerry Springer for hours a day is always bad).
However, this doesn't mean that for one particular child all solutions are equal. In fact, for many children, one solution will be much better than another. There are two separate issues to consider.
First, because daycare is an option, how beneficial it is depends on the available alternatives. In fact, for children from families without many emotional, intellectual or monetary resources, daycare is tremendously beneficial: they are exposed to a far wider variety of experiences and intellectual stimulation than they would be at home. In addition, the staff at a high-quality center may be far more skilled at childcare (in terms of knowledge of child development, emotional responsiveness, and patience) than these parents. For these reasons, programs like Head Start significantly help disadvantaged children, because they provide them with non-maternal care. Here's one paper on the subject.
Daycare being not just a good option, but a better option, is not a fact limited only to the very poorest parents, with the least resources. For example, there is a lot of research on the extremely negative effects of maternal depression on children. If staying at home means a depressed mother (or working out of the home means a happier mother), then daycare would also be a far superior option. In addition, living in poverty has very negative effects on children, even when controlling for other factors (like other characteristics of the parents). If attending daycare means the child would not have to experience poverty, then daycare would again be the best choice.
The other issue is one of individual differences. Daycare is intrinsically stimulating (all that exposure to other children, strange adults, and different routines). Some children will love this (especially extroverted, adaptable types); for other children, however, this stimulation will be very stressful. It seems that boys are more sensitive to stress; also, children with "difficult" personalities (in other words, children who have a hard time adapting to change, are more emotionally reactive/sensitive, and who have less self control) are more likely to be negatively affected by daycare. Here's one paper on the topic. In other words, a cheerful, easy going girl baby/toddler will probably thrive in daycare; a sensitive, moody boy may have a very hard time.
There is reason to believe that these differences are largely genetic (in other words, just part of the child's inborn personality). In fact, there is evidence that genes determine how children react to having bad parents (original paper here). Some children would probably thrive even in a really bad daycare (or at home with a bad parent); others need just the right environment to flourish.
Perhaps in the future, genetic testing will have advanced to the point that parents will be able to make decisions about childcare based partly on their child's unique genetic susceptibility. For right now, though, it's important for parents to holistically consider what skills/weaknesses they have (as in, great skill in dealing with toddlers OR the tendency to get depressed when not working), and also what skills/weaknesses their children have.
There really is no one-size-fits-all solution, and anyone who proposes one is not only wrong, but positively harmful.
Previous posts on this topic here and here.