After spending some time on the trail with the Garmin Colorado 300, I've decided against recommending it for backcountry navigation. Barring some major firmware changes, it's beginning to appear that the Colorado is designed more for geocachers than for hikers, mountain bikers, and the like. There may be more issues, but here are two that caught my eye.
Can't reposition waypoints
When planning a trip, I use mapping software (with topo maps and sometimes aerial photos) to plot out the trek. I'll create waypoints for trailheads, trail junctions and features I don't want to miss. These waypoints, created on my computer, can be off by as much as several hundred feet in the field. So when I get to the junction, etc., I like to use the 60CSx's "reposition waypoint" feature to mark the correct location. That way, when I get home, I have an accurate collection of waypoints for future reference or for sharing. Unfortunately, this feature was left off the Colorado, though it should be easy for Garmin to add.
UPDATE: This feature has been added to the latest beta firmware release.
Viewing multiple tracks
I also like to load tracks representing trails, to follow as I bike or hike. When you download these from a site such as MotionBased (soon to be Garmin Connect), you may have to use several to get coverage of the entire route you are planing to take. But unlike previous Garmin handhelds, the Colorado can only display one track at a time.
Perhaps these are "power user" issues that won't matter to you. But if they do, beware. An older Garmin handheld may be a better choice for backcountry use than the Colorado.