I've been using a Garmin Edge 810 since last spring. I'm happy with what I have now that I've spent enough time with it. While Garmin itself doesn't support Linux (at all), the unit is still accessible in Linux as mass storage device. So as was said above, you can create routes in a browser and save them to the device - just don't rely on Garmin Connect to put them there for you nicely. I guess there's a plugin you could try, also.

I got the package with the heart rate monitor and speed/cadence sensor, so now I'm accustomed to using that data. As far as visible during sunlight - yes. Probably better than a phone. Visible at night? Yes. But the backlight turns off to save battery, so you'll need to tap the screen to wake it up (unless your rigged for charging capability with a hub dynamo).

Garmin did come out with a Garmin Edge Touring over the course of the year for touring cyclists. From the reviews, it is basically a stripped down version of the Edge 810. It is not ANT+ compatible (no HRM, speed/cadence or power sensors), but if navigation is what you're after it is priced significantly lower and is advertised to have a longer battery life.

As far as "bicycle navigation that doesn't suck"? Well, if I plot my own routes and upload them, I'm happy. If I'm just relying on the unit to tell me where I am, I'm happy. I'm not very happy with letting the unit auto calculate my route, though (I don't think it realizes its a "bike" GPS when it comes to choosing roads).