I've never used HKN, so I only know what I've seen discussed here but the big perks of IS/ISBoxer
Instant pip swapping.
Simple keymapping via a GUI tool (ISBoxer).
Virtualize configuration files that allows you to run ONE WoW install and retain settings for each instance (toon).
Native support for G keyboards (and lights to indicate status)
Easy Mouse broadcasting / Stupid easy casting of AoE target spells (Blizzard, Hurricane, Death and Decay, etc)
40 second wizard to get up and running with FTL.
Easily flexible window layouts.
Clickboxer http://www.lavishsoft.com/wiki/index.php/ClickBoxer
PiP Swap speed demo scroll to 2:06
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