Interesting... I wonder if the "versapak" battery is NiMh or NiCad? If it's NiMh it might be worth some serious consideration. Will have to see how much the proprietary charger costs.
I really like the flat discharge rate that I get from my Brink 3AA mods using NiMh's which is also 3.6v). It's almost like a regulated light the way they drive their power then die off so suddenly.