I've tried it for Orange County. and it seems to work all right. For example, I asked for an itinerary between Disneyland and Cal State Fullerton. Google Transit gave three public transit options, plus a link to driving directions.

This is good because I live in Orange County. But what if I live in -- or want to travel to -- Los Angeles County, or another county?

Google Transit only covers about a dozen transit systems. I'm looking forward to when it is more comprehensive.