I have a 305 and use it with Memory Map for navigating.
It’s easy to use and just points you to the next way point and lets you know when you’re approaching a turning point.
It picks up a GPS signal quicker than my Tom Tom and the battery life is at least 7 hours, I’ve never used it for longer though without recharging it, which only takes a few hours through the PC or supplied 240v charger, you can also charge through a car’s 12v socket if you have the right lead.
You can choose what data to display on the two ‘cycle computer’ screens and there’s loads to choose from.
It seems very robust, I’ve had mine for 2 years and it’s been very wet a lot of times and often gets washed with my bike. (I did protect the screen with clear vinyl though). The mounting bracket did break, but as you get two in the box, this wasn’t an issue and to be honest I think I was partially to blame.
However, It can only hold 500 way points and each route can only be 100 points long, which means longer rides have to be broken down into sections, or simplified on the PC.
The altitude reading is via Barometric pressure sensor rather than GPS but seems accurate to within 10m or so, you can do circular rides that show more up than down or vice versa though. But download your ride when you’ve finished, Memory Map never lies!
They occasionally come up at PC World and Halfords for £50 or less, if you can get one at this price you’ll not be disapointed.