Our daughter is in a class this year of 16 kids- 12 boys, 4 girls. It has really been fine. The preschool and her classroom are very thoughtful and nurturing places, and we had full confidence from the beginning that this was a good place for her. We still feel the same way.
Our daughter- who is a very social, very high-energy, very capable kid- is one of the youngest kids in the class- she turned 4 in November, and the oldest kid turned 5 shortly thereafter. We occassionally see some more aggressive behavior than we used to (like throwing toys when angry, smashing a building she's frustrated with), and we don't know if that has been influenced by playing with boys, playing with older kids, or if it's just her own developmental process. When we see that, we set clear limits about what's okay to do at home- we know that her teachers do the same at school- and it has worked out well.
If I were in a similar situation again, at least with this kid, I would not hesitate to have her there.