We are in MCPS with an incoming kindergartener. Based on the people I know experiences have been varied. With certain less common issues (like apraxia) they are terrible. They just don't have the expertise to serve certain kids. Also kids with more subtle issues they just won't serve and you have to get them help privately.
At the same time, I have friends that have been really pleased with their experiences with services for preschool age, including the Preschool Education Program which is a special education preschool program (part day for most of the classrooms but they provide transportation to daycare). I do know that there have been complaints on DCUM that these classrooms are overcrowded - I don't have personal experience with the program so can't speak to that. They have different types of PEP classrooms for different types of needs. X Minds is a great resource for families of kids with autism in Montgomery County and their website has a good summary of PEP:
https://www.xminds.org/preschool-placement