I'm a fogie! A middle aged one! I don't trust the routing capabilities of my Etrex. Don't get me wrong, it's great in the car if you want a mystery trip and you've got plenty of time, ideally about two and a half times the amount you need for the journey, and you want to go by a route you'd never have thought of, and, importantly, you've got someone to read the instructions.
For the bike I'd rather be in charge of where I go and use the Etrex as an electronic list of what I've decided. When I'm on the ride I know that in 1.7km I've got a left turn and that left turn is part of the plan. I've looked at all the options on the map and that's the way we're going.
The other problem with automatic routing is if you've got age related long sight, it's not that easy to read the instructions as they flash up.
It's a shame that in answer to a question asking for advice about which gps to buy there have been so many answers saying "I wouldn't use one". That's no help as it doesn't answer the question. It is very easy to take an anti-gps position in a forum such as this because there's a certain and strong appeal of tradition amongst touring cyclists, Brooks saddles and cotton duck.
I would suggest reading Francis Cook's pages which are a wealth of information about gps, and which I found extremely helpful when I was deciding what to buy and getting to grips with what I bought.
http://www.aukadia.net/gps/index.htm