Hasbro recently released a line called the Fall of Cybertron, which shows the Transformers before they came to Earth. A similar concept was used in the Protoform figures of the first movie line. That however couldn't be seen as the original Cybertronian form universally. FOC is the more honest attempt to show the original Cybertronian forms (If one may remember, Bumblebee, Wheeljack, Soundwave and the Seekers were shown in the first episode of the original cartoon in those forms).
To compare, here's his original look.
(Prime's classic look based on his Kenworth truck mode)
(The really blocky Generation 1 toy)
I like the Fall of Cybertron Optimus Prime because of his new sloped block look. I mentioned before that I like wedge shapes. That's also because I like slopes in the right places. This version of Prime certainly had them. Some may dislike it or find it ungainly. But I like it. In fact, if the Transformers were to go further into the future, their looks would certainly go back to their Cybertronian forms (they after all would be futuristic by our standards today).
I found out later on that there have been earlier renditions of Prime in his Cybertronian mode. One is the War from Cybertron deluxe figure, and another is a War Within figure. However, I like the FOC design the best.