We had a great experience with foster adoption. We asked for a girl from age 2-5, and were matched with our four year-old daughter eight months later. We asked for a low-risk placement, meaning that it was likely to go to adoption because the birth parents were not following their plan (drug treatment, counseling, parenting classes, visits, etc.). We were asked to consider infants, but we had our heart set on a toddler or preschooler. With a newborn or infant, there is more risk (generally) because the birth parents will still have time left to work on their plan. Also, you should know that infants removed from birth parents usually have tested positive for drugs. Many older children in foster care were also exposed to drugs and alcohol prenatally, so you should reseach these issues.
Adopting from foster care can bring special challenges, but can also be amazingly rewarding. You do need to really do your research, and determine which emotional, learning, behavioral, and physical issues you are willing to take on. There is not guarantee that all the child's issues will be known at placement, but it is best to be honest about what you think you can handle. We have dealt with emotional and behavioral issues, but my daughter is thriving and doing great! Best of luck to you and feel free to ask more questions!