The answer to the original question depends upon how you are going to use the device. If you want to just track distance and see how fast you are going, a simple computer (wireless or wired is the way to do). The battery in it easily lasts a year and you don't have to worry about anything. For a few more dollars you can get other features like HR monitor and the ability to download info to a computer. A step up from that are devices like the Garmin 500 and you can see above how people use them. One thing that complicates their use is the devices have to be recharged and forgetting causes you to not have data on the next ride or it goes dead part way through the ride. At the top in are Garmin devices with maps.
You also can use smartphones with appropriate apps. The most obvious disadvantage is you either can have constant view of the data while you ride or you somehow mount them on your bars/stem. If you don't mind the appearance of the large phone sitting there, you run the risk of damaging the phone if you or it falls.
So there's no easy solutaion that covers all the needs/uses.