I like the idea of GPS ones, but battery life is poor enough to discourage me from using them on long tours.

Essential (absolutely must have or I wouldn't bother to have one):
* I most value trip mileage and consider it the main reason for having a cyclocomputer.
* Good battery life

Really nice to have:
* Current speed
* Cadence
* Time of day
* Temperature - I was surprised how nice it was to have this feature

Maybe nice to have:
* Maximum speed
* Average speed

Wireless would be really nice to have, but I have found wireless to be way too prone to weirdness (logging miles when parked near a neon sign and other interference related problems).

Similarly elevation data and turn by turn directions would both be nice if gps units were less battery hungry. If I really wanted to deal with the battery drain I would probably get these functions from my phone. I currently just leave the phone off 99% of the time and only turn it on for brief calls, emails, texts, google searches, or directions in town.

If battery life was a non issue I'd be pretty satisfied to just get all functions from my phone.