If you are talking about pre-school, I started my son in the special ed pre-school program at 3 yrs old, and it was the best thing I could have done for him. Of course every situation is different, and it depends on the school and the teachers. I know it is hard to send your baby off to school at such an early age, but if it is the right environment, I think it will benefit her.
My son is now almost 6, and has been in special ed for 3 yrs now. He has come so far. He is speaking in full sentences, he is reading, and spelling some words. He still has trouble interacting with his peers, but they are working on it.
I am approaching it one year at a time. Although my son seems to be doing well academically, he still has challenges sitting still and he likes to talk a lot out of context, and he does flap his hands. He seems smart, but I am just not sure how he is going to handle being in a mainstream class. But I also don't want to hold him back by keeping him in special ed. So, we may have to look at homeschooling or a private school that has smaller classrooms.
Good luck on your decision and trust your instincts.