I use the Legend HCX on my bike, both as a cycle computer around town as well as routing finding camp sites etc whilst away touring.
The base map is very very basic and only includes major roads but does include the whole world. It is well worth while getting the UK TOPO maps or the Euro Navigator or the open source maps mentioned on another thread. the Euro Navigator and UK TOPO maps are about 1.5GB each and you can only have one set of maps loaded at a time. I have a pair of 2GB micro SD cards with UK TOPO on one and Euro Nav on the other. There is still plenty of room on each card for track data, routes, extra waypoints etc. The maps can be expensive but then there is always warez or the open source option.
I have found that a few of the POI's for my locla area that are included on Garmins own maps are a little out with there positioning. The POI for our local Sainsburys is about half a mile out. I preffer to plan ahead and add my own custom POI's where possible and you can download more form varius web sources.
MapSource is the Garmin software for uploading new maps, routes and waypoints etc to your GPS. You can upload waypoints directly from Google maps, I'm not sure about uploading routes directly from Google. You can also upload routes and waypoints etc form other GPS enabled software like Memory Map.
Be carefull when buying the bike mount. I bought a mount from Amazon that stated it was good for all etrex model. Problem was that the new back cover did not have the cutout for the SD card slot so it did not fit. The replacement back cover was also pretty flimsy and even if it had fitted probably would not seal as well as the orginal cover. Luckily my Legend HCX came with an extra mounting clip that screws to the back of the original cover. All I needed from the bike mount kit was the bracket to go on the handlebars.