Here are my experiences with three high-end computers I tried, the Avocet 50, the Cateye CC-AT 100, and the VDO MC 1.0+. The Avocet 50 has been replaced with the 55 but I haven't seen one yet. All computers support altitude using a barometric pressure sensor.
* Avocet 50
works well when measuring altitude. It ignores small climbs under 10 meters, which makes it very precise. In the Alps and other mountains I have ridden in the displayed altitude matches the posted altitudes, and after returning to the starting point shows the original altitude to within a few meters. It seems unaffected by temperature and weather changes.
Unfortunately it eats batteries. It needs a new pair every six months, and loses all data when replacing batteries. After the batteries are about half-empty, altitude measurement becomes extremely erratic, once I climbed 200 meters while the bike was leaning against a wall. The casing is very brittle, when dropped it can come apart and scatter components everywhere, especially the batteries (they included two spare lids). And this is for the late production model, not one of the first which were completely unusable. I had mine exchanged four times on warranty.
The software is excellent, however. Every function can be reset separately. The combination of features on the display is dubious (why not current speed + trip distance in the same mode?) and it ignores button presses in quick succession, but basically it's well-designed. It has no temperature and no backlight. There is a cadence option.
* Cateye CC-AT 100
It's ugly and clunky, and the altitude measurement is broken. The altitude display jumps by 5..7 meter every few seconds, less often in the "fixed" newer model that also has a trip altitude function. These errors are accumulated in the total altitude gain, which makes for very impressive totals even in perfectly flat terrain. However, with a fresh battery these problems mostly disappear, then it again becomes more unreliable as the battery wears down.
The user interface is stupid, one button is largely unused while the other must be pressed for two seconds to access important submodes. The combination of data on any given display is better than on the Avocet though.
It is also mechanically stable, unlike the Avocet. It does stop working in heavy rain. Counts can only be reset all at once. It has temperature and backlight, but no cadence option. Like the Avocet it doesn't support 24-hour wallclock mode.
* VDO MC 1.0+
This is my newest computer, and it beats the other two hands down. Finally, a reliable altimeter, and it has all sorts of extras like inclination, top and average altitude, average climb, temperature, two bikes, and so on. It also shows lots of data on the screen, and has three buttons to cycle through the alternative fields - one for the usual distance and max/average speed mode, one for odometers and clock, and one for the altimeter modes; 16 modes total. It's very easy to use.
It's also wireless. That's great because rain can't stop it, but it's sensitive to stray EM fields. Recently it counted two kilometers while in my pocket in a computer store, and four kilometers in a subway near where I suppose the brakes are. Another minor disadvantage is that you need to press a button to get it out of sleep mode.
* Casio ALT-6000
This is not a bicycle computer, it's a watch. Actually it looks like a crashed flying saucer. I bought it to see if there is a reliable way to get altitude readings on a tour. It's a failure.
It shows both barometric pressure and altitude, numerically and in a little graph. Pressure readings work fine in a shop window but when you actually wear it the graph dissolves into a cloud of disconnected pixels. The altitude graph is sort of fun to watch, but serious measurement errors throw off the autoranging. Its long-term history may have some anecdotal value. It has a terrible user interface and forgets the graph when switching modes. It also shows the temperature average between your wrist and the back side of the moon. Or something.
Don't buy this. It's an expensive useless toy for kids and has no real value.
* Garmin Legend, Garmin Vista
Lovely, solves the problem, although at a high price. See below.
The first three computers need to have their wheel diameter set precisely so they know the distance that corresponds to one wheel revolution. To measure this distance, make sure your front wheel is installed with the correct tire, and pump it up to the desired pressure. Mark a spot on the wheel with chalk and align it with a mark on the floor. Sit on the bike and ride forward in a straight line for one full wheel revolution, and mark the spot on the floor under the chalk mark. Measure the distance between the two marks on the floor to the closest millimeter, and enter it into the computer. Some computers require some calculation instead of accepting the distance directly.
Garmin now has a GPS bicycle computer named the Edge 205. I haven't used it because although it could be the first of an exciting new species of bicycle computers, it doesn't seem mature. It's bulky, expensive, and lacks the most important feature of a GPS receiver, mapping. It may have some uses in training for athletes.
See also an article on "heart rate Monitor"
