I've been happy with the wireless Vetta V100A I purchased last year. With respect to calculated elevation gain over the course of a ride, I've compared it to other sources of elevation info (an older Avocet Vertech watch, and route mapping with Topo USA 4.0). The Vetta seems to produce the best results of the three (higher than the Avocet which has a higher "trigger" point, and lower than Topo USA which notoriously overstates elevation gain).

It also features a real-time Grade Percentage display. This is interesting, but sometimes gives funny numbers (too high). Over the course of a climb, the Grade Percentage is mostly OK, but you can't always believe the Max Grade number at the end of a ride.

Specialized also makes a computer that does elevation gain, and I've heard some good reviews on it too.

BTW, it's my understanding that GPS units are not very good at calculating elevation gain.