The data from these computers was accurate and reproducible. It was also useful and easy to interpret. However, as time went by companies saw the potential to increase their profits by adding more data, more storage, more features, and more "analysis." Today's cycle computers are overstuffed toys, full of unnecessary functions. These toys are also overly expensive, but provide little value other than entertainment. All come with often overlooked but major hidden costs.
The devices are tricky to install and tricky to use; they are often unreliable and work poorly under real life conditions; the data is frequently lacking or inaccurate and even more frequently not reproducible. Many functions are based on formulas or lookup tables and may not apply to you. And worst of all, battery life is just about non-existent. You are lucky if the device has enough juice for one long ride. Some GPS devices die within six hours.
The new gadgets also have several readouts leading some people to switch between screens instead of paying attention to the road. I have personally witnessed several accidents where riders hit the deck because they were fiddling with their cycle computers. Not only is there a data overload, most devices also suffer from very poor interface design. We may have to wait until Steve Jobs turns his attention to the cycle computer before things will change.
Newer gadgets also allow the user to download values. This ensures hours of additional wasteful entertainment. Given the poor quality of the recorded data, one wonders how useful analysis can be. Some device makers are apparently aware of this problem and they sneakily "correct" the data coming from your gadget. In most cases they do so without warning you or without giving you the option to disable the "feature." Nearly all download programs smooth the data. Smoothing can be helpful but in many instances it is the instantaneous changes that have the highest information content. However, given the noisy nature of the data, it is probably just as well that these are smoothed out.
Some companies go even further. Most GPS devices will correct your recorded position when it happens to lie in the scenery instead of on the road. One can argue that this is a good thing. But what does it tell you about your speed or distance? If these are "corrected" too, then what did your expensive gadget really do for you?
One of the first "hot" features added was the altimeter. Altimeters work with pressure sensors that need to be nulled before every ride. Unless you are a airline pilot with access to air traffic control, that is easier said than done. Blame the interface, or the complex routine needed to reset the device; or the difficulty of finding out what the barometric pressure is at the time of the ride. Whatever the reason, the fact is that most people don't bother resetting their altimeters before a ride. What that means is that their data are off by a sizable margin. Never mind the temperature sensitivity. As the ride progresses and the temperature changes -quite common on the California coast- the meter starts drifting over time, making all readings completely useless.
While most cyclists are aware of these problems, few seem to care.
It gets even better when you add a heart rate monitor. Not only can you pick up other people's readings unless you have a coded transmitter, you can also skip beats or record extra beats due to poor connections. The efficiency changes with skin moisture and can fail with no sweat or too much sweat. Movement of the strap can easily cause artifacts or missed readouts and many devices pickup EMG signals from the chest muscles. The latter is much more of a problem for runners who move their arms than for cyclists who often sit quite still. Even so, about half the time your readout is incorrect or unreliable for one reason or another. And it happens more frequently when you go hard. Needless to say that is also the time when the information is most valuable.
Once again experience shows that most users are aware of the problems but that does not seem to affect their enthusiasm for using the device. Nearly everyone I have talked to told me their heart rate monitor does not work properly from time to time. All have learned to ignore very low or very high readings, and most have developed some "intuition" for correct readouts.
Heart rate monitors have alarms to alert you when you cross a preset threshold. It is quite common to run by someone whose heart rate monitor is acting up and beeping furiously. Much like the neighborhood car alarm that everyone ignores, the owner of the device has long since stopped paying attention to the beeps. But he or she still uses the device in every race.
Ultimately the real problem here is not that devices give random incorrect readings. When used properly these tools can be quite informative. The trouble is that most people don't take the time to learn to use them properly. But they still rely on the "information" or defer to the meter. And in the process they no longer pay attention to their body.
Listening to your body is one of the most valuable skills you can develop as an endurance athlete. And one of the best ways to do so is to train without gadgets. Save the devices for a few tests that you can use to track your progress on a regular basis. At other times, you are better off with just a plain old wired speedometer. It will save weight and batteries too.