I tried Google Maps Bicycle option on some city, suburban and rural routes. There are still quirks in both the maps and the algorithm. I hope some of the issues will be fixed in future releases.
In rural areas it frequently uses gravel roads rather than using low traffic state routes. This is especially true of long distance routes, such as Chicago to Omaha. Over long distances, the routes suggested with the Bicycle option are often the same as one gets when using the Walking option.
It oddly also seems use some very high traffic volume streets passing through some towns.
On the plus side, it frequently offers several routes to pick from, one of which is likely to be reasonable. And it is easy to recalculate a new route by dragging and dropping some points from the initially calculated route.
It was helpful in cutting 10 miles off of a 50 mile trip from Homewood, IL to the Indiana Dunes SP, but it took a lot of manual tweeking to get there.
In both city and suburban areas, there were several odd cases where, when roads intersected, it avoided the intersection by zigzagging thru residential areas a few blocks from the intersection. It appeared to not like to route thru the stop light. In both the city and burbs routes sometimes zigzagged on and off of a perfectly good street, including some streets with bike lanes.
It is currently missing two of the longest bike paths in the state of IL ... the I&M Canal from Joliet to LaSalle (approx 60 mi) and the Hennepin Canal from Bureau Junction to Moline (also approx 60 mi) I presume these oversights will be corrected with time.
I see Google Maps bicycle routing as a useful tool for initial route planning used in conjunction with, but not instead of, state and city bike route maps.