For mine, the 917's mystique is that it was the Porsche built specifically to chase the marques first outright win at La Sarthe; it's generally mentioned that its design was evolutionary rather than revolutionary, but the successes of the 917K in the early 70's cemented it as epitomising that era of sports car racing.
The original design was an aerodynamic drama: "der compooder" said 9"-wide tyres gave the best drag/grip solution, but as Frank Gardner summed it up in not so many words: der compooder didn't have to try and keep it on the black stuff at The Green Hell! I'm sure that I've read somewhere that the moving tail flaps were quite effective, but the rulemakers ordered them fixed... which made it that much more difficult to drive. Of course it wasn't the only issue with them, at least initially... in the 1969 car, FG says they had a gauge on the top of the dash, with a pressure reading (apparently the frame was pressurised), and the engineers told him if it should ever fall to zero - ie the frame cracked - he should drive it slowly back to the pits... to which Gardner replied, no, he would park it immediately and walk back thank you very much.
Frank Gardner has been infinitely quotable on his experiences with 917 from 1969, ake on the thing in 1969 is a real scream (Frank can use a bit of poetic licence when telling a tale by many accounts)... reckoned that the frame flexed so much that if you'd reach for the gear lever as you exited a corner, and it wouldn't be in the same place you left it... at the Nurburgring in 1969, David Piper brought the car in early, told Frank he was too young to die, and left him to it... Frank said that day he drove with levels of concentration that he never knew he possessed; in a private ceremony at the factory the following week, they presented the two drivers with the Iron Cross...
The 917 laid the cornerstone for the Porsche sports car domination of the 70's and early 80's, spawning worthy successors such as the 936, 956 and 962, and while technically it probably wasn't quite Car Of the Millenium, the 917 shape,story and mystique is sufficiently emotive that it will alway gather afficionados.