I have used cheaper car GPS units on the bike, and they might do what you want for not a lot of money. Just get the right mount.
At the time I was beta testing maps and alternative GPS software, so used the bike time to also test it out. But as far as longer term bike used the garmin 660 wins hands down for readability, build quality, waterproof, use with gloves on. I have been in bad weather with car units and could not see the screen due to the rain, and going through unknown parts of Sydney on voice instruction alone was not great fun. But the unit survivrd!
The 660 still has its faults as do every gps I have used. As TJ mentions, I would love it if I could use the garmin hardware with other maps and software (when I was beta testing I could expect monthly updates incorporating comments (garmin does really seem to care) and the rural mapping of addresses was far more accurate. The garmin hardware is what makes it best for me, as I know of other maps and software that could easily do the job.
I also use the 660 in the car as it comes with car and bike kits, and it doubles as a car and bike hands free for use with the mobile.