We used the Google Maps bicycle option several times on my cross-US trip earlier this summer. During that time, the bike route option became available on the Google Maps version available on the Blackberry, as well as in the standard browser interface. I liked the Blackberry version interface a lot, and often used it to find lodging.
Overall, the routing is pretty useful, BUT
1. It doesn't seem to care whether the road is paved or not. I was put on gravel roads in Indiana, Minnesota, and South Dakota. This was amusing in northern Indiana (where I was familiar with the area, and the roads are on a grid system, so we just went one way or the other to get a paved road), but a nuisance otherwise.
2. In South Dakota, we mostly followed old US 16 (parallel to I-90). This is known in parts at state route 248, if I remember the number right. Google maps really did NOT want to let us go on that road and would route us off it at every step in favor of longer roads locals said were unpaved.
3. It did not want to allow us on interstate highways, even in states such as SD, ID, OR where riding on the interstate shoulder is the best route.
4. You cannot reverse a route usefully. I had west-to-east cue sheets for part of the ride and thought I could edit the route from A to B, and then switch from B to A. But it redoes the route when you do that, so you lose your editing.
5. Possibly because the Rails to Trails conservancy was an important cooperating agency, it tends to have a huge bias in favor of trails, regardless of the true condition of the trail. Example: except for the nicely paved eastern section, the Wabash Cannonball Trail in Ohio is so overgrown with bushes and with so poorly maintained a surface that it is basically unrideable (and we didn't see anyone on it for several miles until we gave up and went to Alt-US20).
It's a beta version, and likely to get better.