I use the Garmin Oregon 450 as well. Routing is a hit or miss. It worked once quite well, getting me out of a town on my way and the next moment it tried to send me down interstates and gravel roads. Much better is if you have created a track of the route you want to cycle. You can show the track on the map and see which way to go. Of course it doesn't give you audible alerts. The downside with tracks is that you have to create them before hand. That's time intensive and keeps you on a fixed route (i.e. not easily updatable on the road).
The Oregon 450 really shined when I did night riding. I knew exactly where I was in the dark and could track progress easily. A set of alkali batteries is good for a day's worth of riding (~8h?) have spares if you do a long day. BTW, the Garmin bike mount work quite well and is pretty cheap (~$8-$10).