I've never used a Garmin or similar mapping GPS outdoors, but I've looked at them in the REI store and read reviews which complain about the display performance outdoors. These are not high-contrast AMOLED displays like on a good smartphone. Have you ever used a modern smartphone like the iPhone 4? The Kindle and other e-readers is a different story. E-ink really does work better in sunshine. But that's irrelevant since e-readers would suck as mapping gps's due to the lack of pinch to zoom.
I'm having a hard time understanding why you need GPS turned on 10 hours/day. First, I don't want to ride 10 hours/day. Second, most of the time when I'm riding (or driving in a car for that matter), I'm just following the road and I know which turns to take. There simply aren't that many turns to take outside of cities. I'm certainly not going to be staring at my GPS for 10 hours/day. So the only time I consult the GPS is to plan my route and refresh my memory at critical intersections. 20 minutes a day for that is plenty. Unsuspend the phone, look at the map, maybe pinch and zoom to see the context, then suspend the phone again--takes about 30 seconds total, at max. If I need GPS to find my current location, that 30 seconds becomes maybe 2 minutes. 10 such uses/day should be more then sufficient for the areas I tour.
Maybe you plan to rely on your Garmin mapping GPS as your sole map. In that case, battery life would indeed become an issue. I would never rely solely on a mapping GPS myself, due to the possibility of electronics failing outdoors. Also, big paper maps always give a better overview than small displays. But even with a mapping GPS as your sole map, I still don't think you'd need 10 hours/day of staring at the map. Maybe 1 hour/day max. It is true the GPS drains the battery faster, but then you don't need the GPS turned on most of the time. Most of that 1 hour usage/day would involve panning around to plan your route--the sort of thing I prefer to do with a big paper map. Both the Nokia N8 and iPhone will give at least 10 hours of usage in offline mode. So all you have to do is stop in at a motel, campground, cafe, laundramate or other facility where you can borrow electricity more frequently than once every 10 days.
I do bring along a separate Garmin Foretrex 301 non-mapping GPS, which is waterproof and has 12 hour battery life. As with the phone, I certainly don't keep this device switched on all day. Rather I only use it when the phone won't work (such as in deep canyons or under trees, due to lack of a SIRF chip) or when it is raining (since my phone doesn't have a waterproof container, though these are available). Like I said, I find it difficult to get lost when sticking to roads, as I do when cycling, even dirt roads not shown properly on my paper maps or the smartphone maps. Hiking is a different story, and then I use this GPS more frequently (maybe 3 times a day on the Pacific Crest trail in some areas where trail junctions are poorly indicated).
Here is that link to the guide to using the iPhone as a mapping GPS (I'm repeating this because everyone should read this page who has any interest in the subject of mapping GPS's, whether or not you use an iPhone):
http://adventurealan.com/iphone4gps.htm