I have spent a few minutes testing Google Maps directions for bicycles. What it's doing in my area seems to be the following.
1) Whenever possible, use a separated bike path
2) When that's not possible, use a bike lane
3) When that's not possible, use a street
which results in directions that pretty much follow the way I tend to do things for really big trips that are primarily N/S -- for those trips, it routes me up the San Gabriel or LA River bike paths, and this is what I'd do. I am not afraid of riding on the street, but the LA and SG River trails are like bike freeways. You can ride on them full-speed for as long as you can handle with hardly any traffic. Why ride in the street and deal with traffic when you don't have to?
But E/W gets more interesting. For instance, if I do directions from my mom's place in Huntington Beach to my home, it is exactly how I'd do it (Garfield to Seapoint to PCH to 2nd) until it gets to Belmont Shore. Then, seeing that there's a segregated bike path along the beach, it sends me down that way, rather than up a perfectly quiet street like 1st or 2nd. That adds real distance to my trip where it really makes little sense.
It would be helpful if it let you decide how agressive it should be on choosing separated paths vs bike lanes vs surface streets, so people could get maps more geared towards their preference and comfort level.
That being said, this is a new tool, and it works pretty well already. As they get feedback and tune things up, it should prove to be a really useful tool.