I wanted to travel to a friend's house who lives just off Braddock Road in Springfield VA about one mile west of the Capitol Beltway (I-495). I live in Arlington VA not far from Seven Corners, about 8 miles away. I had never been to my friend's house before, so I asked Google to give me cycling directions there. What I look for in the directions is how it overcomes the major obstacles, in this case, Route 50 and the Beltway were two major roads I had to cross. Google did not have me cross Route 50 at the new pedestrian overpass at Seven Corners, which I think is a better option than what it showed me. It did, however, have me cross the beltway at the pedestrian overpass that is north of Braddock Road about 1/2 mile, which is definitely the best option. All in all, the directions were not bad. A less experienced cyclist might have found some of the roads--particularly Braddock Road--a bit daunting to ride on. However, the directions worked for getting me there.
All in all, Google biking directions is a useful tool, and I expect it will become more useful over time as Google--and users--improve it.