zigzterIIRC, I started watching around season 5 of ESEA (Leogeo & Schetter <3).
I think I stumbled on a Top10 plays from eXtv and saw YouMustRecover in it, who was a semi-regular on the pub server I always played on, which made me research this whole "comp tf2" thing.
I didn't actually start playing it myself until season 10 though..
ymr pretty much got me into competitive as well. i started doing steam group pugs around s7 or so but didn't really get serious about it until s9.