File types and UI experience (fluid animation etc) are the two biggest things I look for in extenders.
Specifically, outline if you can stream direct DVD rips, Blu-rips (.mts) or .mkv rips etc.
Any plugin issues with the extenders? Does Netflix work?
Your source for everything home theater & HTPC related
I would echo what Alan has said.
1) Fluid Animation
2) File types (divx, xvid, mp4, mkv) and how well they work if they do
3) Plugins (definitely Netflix streaming)