I've had a chance to test out a new pannier system for my convertible bags, and I want to share my results.  I posted about this a while back here.  I'll now use a modified hook & bungee system as the standard setup.  The bungee unhooks from one side, and easily tucks into a side strap for use as a backpack.  It does require an extra couple seconds to convert the bag, but the pannier setup is very secure, and ideal for commuting, touring, whatever.

woodward backpack pannier

As an upgrade (for $25), I'm still offering the Ortleib hook setup.  The advantage with these is the brilliant spring loaded clip which secures the bag to your rack with no bottom hook needed.  I've heard a lot of concern about the wobble factor of this setup (one of the reasons I developed the new system) but it works very well for commuting purposes, and converts very quickly.