I can't say I do it for thrill, that's what roller coasters are for. But I do it mostly for hobby (like to tinker), practicality (if the performance is there, why not use it), and lastly price (I feel much better about the possibility of destroying $100 parts vs $500 parts while tinkering). So I guess it all comes back to tinkering as my primary motivation.
I generally don't find much fun in taking top of the line stuff to lofty levels. I much prefer taking someone elses scraps, tweaking them, and making them perform way beyond their capability.
For example, in last year's hardware drought, I bought about bunch of old 370 boards and cpus to tinker with. Low and behold, I was able to slap together several tualeron systems running above 1600mhz, performing in the XP1800+/P4 2gig range, and all of it ran very quietly.