CT is the only system I ever used when I played live. I remember reading Book 5 and was so enchanted, I spent the whole night in the jacuzzi reading it. Water damaged the copy I borrowed so bad, I paid for a replacement from the lender.
I have read and enjoyed MT, reading the three core books repeatedly. I never used the task system for skills, instead using it for metagame management and how much information I should reveal to characters during an adventure. Also, despite being exposed to High Guard and Striker, for some reason I cannot comprehend MT vehicle design. Is it because I am too old?
TNE is an occasional good read, though for some reason it took me forever to get their task system through my head.
T4 was interesting, though the 'roll low' system made my head hurt. Good advice on running adventures, though.
Never had the money or access to T5 or Mongoose, although the occasional crunchy bits I have seen here look VERY interesting.
Got the same excitement as in reading Book 5 when reading TNE FFS, but I discovered I was internally converting everything to CT stats.