...Is it worth it to ...ie make a copy of my entire HD?
To coin a phrase, 'it's never worth it till it's too late'. Whether it's worth it or not depends on what price you put on your peace of mind.
I bought an external la cie after a catastrophe with my internal. Not a catastrophe in that I lost anything but it took me about a week to fully rebuild everything and have it all humming along nicely.
With SuperDuper you can, with the press of a button clone your entire HD onto the La Cie and it will also be bootable as well which means you can take your La Cie drive (if it's firewire) and plug it into another mac and boot up from it and lo and behold you will be looking at your computer with everything exactly how it is.
Even if you haven't 'lost' your original software CD's, it can be very time consuming to restore everything as you will probably have to install updates and maybe a bunch of addons.
Also you can install system software updates with complete confidence. If something goes wrong with the update or if indeed the update is corrupted, instead of having a major problem you have no problem at all, you simply boot up from your external, then select SuperDuper and clone the external back to the internal and wa la, you are back to where you were before the crash.
Make two partitions on the external with DiskUtility, one as big as your internal and the other the size of what is left over to store extra stuff.
When using SuperDuper use the option to "Backup all files", not the sandbox.
Complete peace of mind against any disaster, is it worth it? You decide.