There's two GPS options. A phone with GPS is inexpensive, but the batteries don't last too long. The Garmin bike computers are costly, but they are designed for bike rides, with a small size and a useful display, that works great in full sun or at night. They'll run well over 12 hours on a charge. And the Garmins are waterproof!

I got my Garmin so I could add routes and use the Garmin for navigation. It shows the route as a purple line on the map, and can beep and show prompts for turns.

I found that I like the recordings of my rides. There's a lot of different software and web sites to upload the ride.
Here's a few screen shots of ride data:

The free My Tourbook software totals my distance, elevation, etc by month and by year. It can map and graph each ride in a lot of different ways.
The overview display. Altitude graphed in green, speed in blue.


A ride's route with color coded speed:





Rides can be uploaded to ridewithgps.com showing the map, and a lot of statistics.
No other site has this graph: Minutes spent at each mph speed, then average grade at each mph. (this was a hilly ride!)