One question I have, is can I use my handheld GPS that I use for GeoCaching to assist me as a bike computer since it will track speed, distance, route, etc? Or are there features that a bike computer will have that are necessary for me to understand what I'm doing that my GPS won't offer. I should add that mine is a Garmin eTrex Summit HC gps unit.
I think your eTrex Summit has all the functions of a basic bike computer. A basic bike computer provides the following info:
Current Speed
Overall Odometer - generally not reset, so it shows total miles since installation.
Time of day
The following easy to reset trip registers:
Trip Odometer
Trip Moving time
Trip Max Speed
Trip Mean Speed
.
The Trip Computer page of your eTrex Summit shows all of the same info except Time of Day, which, IIRC, is available on another eTrex page.
Some up scale bike computers can track elevation, which your eTrex also does.
Some up scale bike computers track trip Time/Speed/Distance for two separate trips, which is handy when following cue sheets with instructions given in terms of leg distances. The eTrex Summit can only track TSD for one trip at a time. You could reset the trip odometer at the start of every leg to get the same info, but then you would loose the trip average speed and such.
Some up scale bike computers track cadence. As Hack pointed, other bike specific GPS units can track cadence and heart rate, but the eTrex Summit does neither.
The eTrex Summit, along with most other GPS units, can record tracks and routes. Bike computers do not.
A really nice feature of the eTrex on a trip is that, if you loaded the Garmin maps, it can help you get unlost when the road names on the ground don't match the road names on the map.
On trips, but not generally on local rides, I use a eTrex Vista HCx with the Garmin maps. In addition to getting me unlost, I use it to find the nearest services ... things like campgrounds, hotels, restaurants, convenience stores, gas stations, or whatever. I've also used the mapping capability to find my way around detours.
[Edit]There are other features of the Garmin eTrex series GPS that a touring cyclist would find useful ... such as compass, time and distance to waypoints. Tracks and altitude profiles might be of interest to some.