This is the latest version of the Cx from the eTrex range of navigators. It has a better satellite acquisition time and battery life than previous versions (18 hours on fresh high-power NiCad batteries, less with environmentally friendly rechargeables), plus a new expandable memory (64MB microSD card included) and a bright, full-colour screen.
The SD card allows you to buy into Garmin's TOPO (stripped-out Ordnance Survey data) maps by region or country and have that info directly in your GPS, although you'll still want to use Memory Map or similar mapping software to get the best out of your unit.
However, Garmin push the much more svelte and bike-specific Edge series for bikers, which have better GPS receivers (Sirf3), better mounting hardware and even a heart rate/cadence option on the 305 model. The Edge also isn't much use for navigation on the fells and it's only really at home on the road, where it's a great training aid/bike computer.
With this in mind, the CX is as good a unit as you can currently get. It has lots of information to offer, can increase location confidence (together with the obligatory map and compass) and it'll improve your ride flow on unknown terrain.