When purchasing trades, Omnis, Absolutes, etc. I maintain the mentality of purchasing blu rays of previously owned dvd copies:
My criteria is
1 - The content - am I purchasing for the effects (ie art) or story?
2 - Is the reproduction (sound, image - for comics - reproduction of art, extras, etc), the upgrade really worth the extra money?
With this in mind, additionally any criteria you may have, what collected editions exceeded your expectations?