The Avocet 50 was released about 20 years ago, and built on Avocet's very popular line of computers to include a barometric altimeter that counted up elevation gain while you were riding. (It also came in a very cool package that, as I recall, won some awards for originality. Unfortunately, the package ours came in is long gone.)
The 50's elevation measurements are still the gold standard for most cyclists--by comparison, both GPS units and online mapping programs tend to overstate elevation gain. The 50, which only starts counting when you increase altitude by 30 feet, gives a more reasonable "gross elevation gain" reading that most cyclists can relate to. If you're on a ride that everyone agrees has 5,000 feet of vertical gain, that figure was likely generated by a rider using an Avocet 50.
Using the Avocet 50 is dead simple. There are two buttons on the top that control everything, including setup (which took us about 5 minutes). A simple, two-line display with large-ish digits provides you with info.
The Avocet 50 was also unusual in allowing you to reset maximum or average speed during a ride without losing other measurements. For instance, you could reset max speed after a big downhill to get a new max reading on the next big descent. We have never seen another computer that lets you do this--to this day, every other computer we've used resets everything at once.
The main unit is waterproof and temperature compensated (to keep the altimeter accurate when the temperature changes). It has a wired sensor that attaches to the front fork dropout (a rear wheel sensor was also available). A pair of watch batteries would power the 50 for more than a year.
The 50 did have some issues. Some early units ran through batteries quickly, and the twin battery covers on the bottom had a tendency to break. You could get replacements back in the day, but Avocet's web site today lists almost everything to do with the 50 as "out of stock."
But those were minor concerns. For ease of use, large display, and accurate elevation gain measurement, the 50 has never been beaten.