After much scrambling around the back of the devices, peering through dimly-lit reading glasses, I managed to get the model numbers. 

Installing the software, I'm asked to create an account on the Logitech web server – presumably this will permit me to update the Harmony anywhere in the world.  Once done, I am invited to identify my devices – that's easy, thanks to my previous scrambling (wishing that every manufacturer would write the model number clearly on the front!).  Fortunately, the Logitech servers agree that they know all about my devices; even the geriatric TV.

Next, I'm invited to define activities.  I choose to create "watch TV," "play a DVD" and "Play a VHS tape."

These are relatively simple to create and mostly successful. 

Unfortunately, it's the 'mostly' that exposes the shortcomings. 

Most AV device remotes have an on/off button that toggles the power state of the device.  Unfortunately, if the Harmony guesses wrong, it sets the power state of a device the opposite of what it should be.  Sure there is access to the raw remote buttons but this seemed to happen more often than I'd like. 

Alternately, there is a 'help' function that is available after each activity selection which allows the user to re-activate each subcommand should any of them prove erroneous.

Similarly to the on/off issue, I discovered a personal problem - my DVD/VHS player seems to need a press of the 'input' button to revert to TV only after playing a DVD; if I stop a tape, it happily reverts to TV without intervention.  This is a problem for the Harmony.  The only way I found to get to TV after a DVD is to momentarily play a tape (assuming one is in the player, of course).