So I was wondering, has anyone actually used this quality in a game yet? Whilst I have several good ideas forming for characters with this ability, I haven't been able to use it on a PC so far. Was wondering if anyone out there had used it yet and how it had well it had worked in-game.

And as a second question, assuming that you had to take it, which would you choose? Spell or spirit, and then what type of spirit or spell would you take?

I'd take a spell knack for trid phantasm. I'd also try to get my GM to let me spend karma to increase my Magic but only to the extent that it enhances my knack (maybe you can in the RAW, I don't remember). Since trid phatasm can basically be used to perform all other illusion spells (with applied creativity), it seems like the most bang for your buck.