I have GPS in my Windows Mobile 6 phone, I can use it with either Google Maps Mobile or DeLorme Street Atlas, either the built-in GPS which is uite slow to acquire a fix, or with a separate Bluetooth GPS receiver, which is fast but another box to carry around.
Neither is even remotely as useful as the Garmin 2610 on the handlebar mount.
Google Maps Mobile is almost useless for routing, other than a start and end point. DeLorme is actually rather good, as long as you do it on the PC or laptop first, and copy the routes into the phone.
The phone has audio prompts while tracking a route, but it doesn't give them over Bluetooth so I have no way to get them into my helmet. There's a set of wired stereo earbuds that came with the phone, haven't tried them with this.
I've used the DeLorme for years in the car, and it's great. On the bike, the only way I can use it is to stop and take a fix, see where I am if I'm lost. I don't have a mount for the phone, which might help, but the screen's too small to use like that, and no good in bright light, either.
Out in the world, I can't route with just the phone, I need to use the laptop and download the generated route. The phone is just too slow. It might route me from town to town, but it takes forever, and I can't drop waypoints or vias to adjust the route. The 2610 will route whatever I need right in the unit if I come up with a need for an alternate plan from what I had when setting out.
That said, the Garmin PC software sucks royally, but DeLorme will make files it that MapSource can read to get them into the 2610, and there's a method used by others to get from Microsoft Streets and Trips into the Garmin. MS S&T is not available for Windows Mobile any more, by the way.
I'm working on a comparison "test" of Streets and Trips, Street Atlas, and MapSource, applied to both a Garmin GPS and to a mobile device like PocketPC or Windows Mobile. It'll be a while, though, but hopefully that might help somebody in your position in the future.
Bottom line, don't try to use the phone for GPS instead of a real unit. I tried, it was dismal. Much harder to use than in the car.